How is a technique categorized as a Kekkei Genkai? During the Land of Waves arc, Kekkei Genkai is defined as techniques which are unique to a clan and passed down through the bloodline. After timeskip, it is touched upon again, and is now defined as a technique which combines two elements. Looking back at the Land of Waves arc, Haku's Ice Release satisfies this definition as well. 

However, Sharingan and Byakugan are also called Kekkei Genkai, although they do not combine any elements. Later, Terumi Mei is showed to use Lava release and Boil release, and they are both considered Kekkei genkai, although it appears that this is something unique she achieved, not passed down through bloodline.

Thus, the term kekkei genkai seems to be used in various contexts to mean either:


Bloodline techniques passed down through generations, but not involving combining basic elements (Sharingan, Byakugan users)  
Techniques combining basic elements, but not passed down through bloodline. (Terumi Mei, and a few others)  
Combining basic elements and passed down through bloodline (Haku)


What exactly does it mean?

Edit It looks like people have not understood the question. Let me try again.

Does kekkei genkai mean:
A. techniques passed down through blood.
B. techniques which combine basic elements.
C. Both A and B.

The answers as well the commenter below seem to have assumed "A". To make it even easier, here's a related follow-up question: "If Naruto learns Lightning Release, and then combines with his Wind Release, would that be kekkei genkai or not?"
Quoting from the Natuto Wiki



  Kekkei genkai (血継限界; Literally meaning "a technique limited to
  inheritance by blood" or "Bloodline Limit") are abilities passed down
  genetically within specific clans.
  
  
  Kekkei genkai abilities that work via the user's eye are called
  dōjutsu.
  Other kekkei genkai include mixing one type of elemental chakra with another, creating a new one unique to the users, which is usually impossible for normal ninja, or other bodily manipulations that are usually unachievable by normal standards.
  


  Kekkei genkai and their related techniques cannot be taught to or copied by others. However, kekkei genkai can be given to others
  
  
  Dōjutsu, or other kekkei genkai isolated to a single organ, can be transplanted into the body of another, as was done with transferring Obito's Sharingan to Kakashi.
  Chakra elements can also be passed down to people outside of the clan, such as Yamato being subjected to DNA re-modification by Orochimaru who wanted to acquire the Wood Release kekkei genkai. 
  

Some anomalies in the transfer of kekkei genkai.


  
  Although kekkei genkai are usually genetically shared within a specific clan, sometimes it is unique to one person alone, and that even family relatives do not share it, such as the Wood Release. 
  Yet other kekkei genkai seem to appear in individuals with no known relation, such as the Lava Release, which has appeared in ninja from Kirigakure, Iwagakure, and Kumogakure, while Magnet Release, which has appeared in ninja from both Kumogakure and Sunagakure.
  There is an advanced variant of kekkei genkai known as kekkei tōta, which is a combination of three different nature transformations. (This technique is used by only two shinobis, namely - Ōnoki  and Mū and they too do not share any blood relations.)
  





Using the above quotes as a base for my explainations:

The general meaning of kekkei genaki is an ability that is unique to a clan. 

Kekkei genkai can be classified into two broad categories- 


Abilities that originates from some body organ, eg sharingan, byakugan (only eye abilities have been shown so far) are called dōjutsu. As the wiki rightly mentions these abilities can be transferred by transplanting the organ from the original bearer to a host, eg Kakashi's sharingan from Obito and Ao who uses the byakugan.
Abilities that uses originates from chakra manipulation. Eg, Haku's Ice release, Kimimaro's Shikotsumyaku. The Nara, Yamanaka and Akimichi clan's (to name a few) kekkei gengai uses a yin-yan chakra manipulation for their techniqies.


The above two categories are seen to be transferred within the clan.

However, there are a few exceptions to the general thumb rule.


Some kekkei genkai like the first hokage's wood release is unique to a singular person. 
Some kekkei genkai like Mu's dust release (technically called as kekkei tōta, as it is an advanced form of kekkei genkai) can be taught as in it was taught to Ōnoki. But these are the only two shinobi's known to be able to use the dust release technique.
Certain kekkei genkai seems to appear within individuals that have no relation to each other or without teaching. Examples are: 


  Lava Release, which has appeared in ninjas from Kirigakure, Iwagakure,
  and Kumogakure, while Magnet Release, which has appeared in ninja from
  both Kumogakure and Sunagakure.

There may also be shinobi's that have more than one kekkei genkai, eg Mei Terumī who can use three elements, namely earth, fire and water. However, unlike Mu and Onoki she can not combine the three at once. She can combine two of them at a time and create two different kekkei genkai, namely Boil Release (water + fire) and Lava Release (Earth + fire).




Based on the above categorization we can safely deduce, kekkei genkai means techniques passed down through blood, techniques which combine basic elements. But as discussed above there are a few exceptions.

Now to answer your second question, "If Naruto learns Lightning Release, and then combines with his Wind Release, would that be kekkei genkai or not?" - I'd say yes, as combining lightning release and wind release would put naruto in the exception category where he doesn't gain the ability from his clan, but is unique to him. (However its most unlikely that lightning and wind would make a stable combination.. :P )