What is the Universal Mind? As far as I know, the concept of the Universal Mind (一心, yixin) was first coined in the 'Awakening of Faith'.

In studying Zongmi, Yanshou, Chinul and Wonhyo, I found different translations such as: Universal Mind, All-encompassing mind, One Mind, One Nature, Pure Nature.

Some say it is synonymous with mind-nature (xinxing).

But neither alternative translations nor synonymous tell me what it is.

In studying Yongming Yanshou systematically, I have come to identify what he says it is, its characteristics, and its functions. The issue is that it ends up being identified with so many phenomena that it becomes confusing. 

Its entity:


The dharmadhatu
The mind of suchness
The mind of arising and ceasing
A mind that is unperturbed and enters the dhyanas
A mind that arises in dependence upon practicing concentration and contemplation
The mind that is transmitted from a Chan patriarch to another patriarch


Its characteristics:


The origin of nature and characteristics
The basis of all things
A phenomena in which pure and impure, samsara and thusness intersect
A principle that unifies all the teachings of the buddha
The great tenet
The grand progenitor
The source of all truth
‘Universal’ in that it includes mundane and supermundane dharmas.


Its function: 


Illuminating the myriad dharmas as if reflected in a mirror.
Entering the dhyanas.
Producing prajna.
Perfecting [the actualization of] cessation.
Perfecting observation.


The best explanation I have found so far explains that the One Mind is identified with so many phenomena because it is all of them. Do you know of a more more detailed account, a clear explanation, or an authoritative reference? In 'Wŏnhyo’s Theory of One Mind', Eun-su Cho says:


  The One Mind is an evolution and merger of the concepts of
  ālaya-vijñāna and tathāgatagarbha. It is a well-known fact that the
  ālayavijñāna and the tathāgatagarbha originate from different branches
  of Buddhist thought. The womb of the Tathāgata is a concept developed
  from the optimistic idea that any sentient being has the possibility
  of becoming a Buddha, a concept already described since early Buddhism
  as the “innately pure mind.”


And:


  The One Mind is at times equivalent to ālayavijñāna, to the
  tathāgatagarbha, or to original enlightenment, but its characteristics
  contain the attributes of all. The One Mind represents an optimistic
  and all-encompassing philosophy of the mind, surpassing both
  ālayavijñāna and tathāgatagarbha. Additionally, the One Mind possesses
  the meanings of unity and concentration.
From Zen Master Chinul's 'Straight Talk On the True Mind', ~1205 CE. Translated by Robert E. Buswell, Jr.


  ...
  Question: We have already given rise to right faith, but we are still uncertain what is meant by “true mind.”  
  
  Zen Master Chinul: To leave behind the false is called “true.” The numinous speculum is called “mind.” The Suraligama Sutra sheds light on this mind.
  
  Question: Is it only named true mind or does it also have other appellations?  
  
  Zen Master Chinul: The names given to it in the teachings of the Buddha and in the teachings of the patriarchs are not the same. First let us explore the teachings of the Buddha.  
  
  In the Bodhisattvasila Sutra it is called the “mindground” because it produces the myriads of good dharmas.  
  
  In the Prajnaparamita sutras it is referred to as “bodhi” because enlightenment is its essence.  
  
  The Avatamsaka Sutra names it the “dharmadhatu” because it interpenetrates and infuses all dharmas.  
  
  In the Diamond Sutra it is called “tathagata” because it does not come from anywhere.  
  
  In the Prajnaparamita sutras it is also referred to as “nirvana” because it is the sanctuary of all the saints.  
  
  In the Golden Light Sutra it is said to be “suchness” because it is true, permanent, and immutable.   
  
  In the Pure Name Sutra it is named the “dharma-body” because it is the support for the reward and transformation bodies.  
  
  In the Awakening oj Faith it is termed “true suchness” because it neither arises nor ceases.  
  
  In the Mahaparanirvana Satra it is referred to as “Buddha-nature” because it is the fundamental essence of the three bodies.  
  
  In the Complete Enlightenment Sutra it is called “dharani” because all meritorious qualities flow from it.  
  
  In the Srimaladevisimhananda Sutra it is named “tathagatagarbha” because it conceals and contains all dharmas.  
  
  In the definitive sutras it is named “complete enlightenment” because it destroys darkness and shines solitarily of itself.  
  
  As Zen Master Yen-shou’s Secrets on Mind-Only says, “The one dharma has a thousand names: its appellations are each given in response to different conditions.  
  
  The true mind appears in all the sutras, but I cannot cite all the references.  
  
  Question: We now know what true mind means in the teachings of the Buddha, but what about the teachings of the patriarchs?  
  
  Chinul: In the school of the patriarchs all names and words are severed; not even one name is sanctioned, let alone many.  
  
  In response to stimuli and according to faculties, however, its names are also many.  
  
  Sometimes it is referred to as “oneself,” for it is the original nature of sentient beings.  
  
  Sometimes it is named “the proper eye,” for it makes visible all phenomena.  
  
  At other times it is called “the sublime mind,” for it is empty yet numinous, calm yet radiant.  
  
  Sometimes it is named “the old master,” for it has been the supervisor since time immemorial.  
  
  Sometimes it is called “the bottomless bowl,” for it can survive anywhere.  
  
  Sometimes it is called “a stringless lute,” for it is always in harmony.  
  
  Sometimes it is called “an inextinguishable lamp,” for it illuminates and disperses delusion and passion.  
  
  Sometimes it is called “a rootless tree,” for its roots and trunk are strong and firm.  
  
  Sometimes it is referred to as “a sword which splits a wind-blown hair,” for it severs the roots of the defilement.  
  
  Sometimes it is called “the unconditioned kingdom,” for the seas are calm there and the rivers clear.  
  
  Sometimes it is named a “wish-fulfilling gem,” for it benefits the poor and distressed.  
  
  Sometimes it is called “a boltless lock,” for it shuts the six sensedoors.  
  
  It is also called “a clay ox,” “a wooden horse,” “moon of the mind,” and “gem of the mind.”  
  
  It has such a variety of different names that I cannot record them all.  
  
  If you penetrate to the true mind, you will fully comprehend all of these names; but if you remain dark to this true mind, all names are only a block.  
  
  Consequently, you must be precise in your investigation of the true mind.  
  
  ...   
  
  If we have faith in this, we should realize that all the bodhisattvas of the three time periods are studying the same thing -- this mind. All the Buddhas of the three time periods have the same realization -- the realization of this mind. The teachings elucidated in the tripitaka all elucidate this mind. The delusion of all sentient beings is delusion in regard to this mind. The awakening of all cultivators is the awakening to this mind. The transmission of all the patriarchs is the transmission of this mind. The search in which all the patchwork[-robed] monks of this world are engaged is the search for this mind.  
  
  ...  
  
  
    The mind: it is vacuous, empty, sublime, and exquisite; it is clear, brilliant, numinous, and bright. It neither comes nor goes, for it permeates the three time periods. It is neither within nor without, for it pervades the ten directions. 
  
  
  ...  
  
  This essence is not only the Buddha-nature with which all sentient beings are innately endowed but also the basic source of creation of all worlds.
  
  ...  
  
  
    The wind moves but the mind shakes the tree.
  
  
  ...  
  
  
    Those who are aware of it know that it is the Buddha-nature; those who are not -- call it soul or spirit.
  


To me, this sounds synonymously with Rigpa, Awareness of the Ground.

If you're looking for a straightforward contemporary explanation here is mine: The Universal Mind is Information at large. Or more accurately, it is the informational aspect of the energy flow in thermodynamically non-equilibrious systems.

Although, in my personal opinion, all these reifications of the absolute are dangerous and misleading. This is why I prefer the supra-conceptual teaching style of Zen and Dzogchen/Kagyu, because it turns your eyes to the moon, instead of talking about the fingers.