Why is the IUPAC definition of adsorption so different from other definitions? NB: This question has been edited. Originally, some of the confusion was caused by me not knowing that condensed phase could refer to solid phases as well. This has been removed, but the confusion caused by the other elements still remains and even the first answerer to this post was unable to clear that up.
The Gold Book's definition of adsorption

An increase in the concentration of a dissolved substance at the interface of a condensed and a liquid phase due to the operation of surface forces. Adsorption can also occur at the interface of a condensed and a gaseous phase.

What I've learned is that adsorption is when a fluid, or the solute(s) within a liquid, adhere to a surface. The IUPAC definition however, only recognizes the case when the solute(s) within a substance adhere to a surface. A more limited, and to my knowledge, less useful definition. Furthermore, the formulation is very ambiguous. It does not state if it is the condensed phase or the fluid phase that contains the solute(s), or if either can contain the solute(s). The middle option would necessarily be in contradiction to the IUPAC's definition of a solution:
The Gold Book's definition of solution (shortened):

A liquid or solid phase containing more than one substance, ...

This definition does not count gaseous phases as solvents. The fluid phase can be a gas, but if the dissolved substance is always within the fluid, then in cases where the fluid phase is a gas, it would be a solvent gas. This is assuming that dissolved substance is the same as solute.
Now, for part two of this mess:
Furthermore, the IUPAC definition of absorption encroaches onto the semantic territory of adsorption, at least the territory that I've seen and been taught.
The Gold Book's definition of absorption

The process of one material (absorbate) being retained by another (absorbent); this may be the physical solution of a gas, liquid, or solid in a liquid, attachment of molecules of a gas, vapour, liquid, or dissolved substance to a solid surface by physical forces, etc.

The last part there is the, or a part of the, definition of adsorption. It described surface attraction, which I thought was clearly within the domain of adsorption. The first part of the definition says that the absorbate and absorbent create a physical solution. I cannot find any definition of that, but I assume they are referring to a homogeneous mixture where the solute isn't disassociated, but still kept within a solution complex. However, an absorbate isn't (necessarily) homogeneously distributed within the absorbent. This means they're violating their own (and countless other) defintion(s) of solution.
As mentioned, a solution is a single phase containing more than one substance, and a phase, as defined by the IUPAC, is homogeneous.
The Gold Book's definition of a phase:

An entity of a material system which is uniform in chemical composition and physical state.

Are these IUPAC definitions outdated? If so, what is the most authoritative definition of adsorption and absorption. I like to use words consistently, but given how inconsistent and/or ambiguous scientific literature can be, I want to pick the best definition in order to influence the literature, if however minutely, towards more consistency and away from ambiguity.
The IUPAC defintion isn't more limited than the ones you've read. Instead, it is more general.
Your confusion results from not being familiar with the term &quot;condensed phase&quot;. You think a condensed phase is a 'more condensed liquid', and consequently think IUPAC is referring to a surface formed between a liquid and a 'more condensed liquid':  &quot;where there is an interface of two liquids, one containing a solute, and one being more condensed&quot;.
No.  A condensed phase means a liquid or a solid.  Thus, when IUPAC says the following, what they mean is that adsorption can occur at liquid-liquid, liquid-solid, gas-liquid, and gas-solid interfaces.

An increase in the concentration of a dissolved substance at the
interface of a condensed and a liquid phase due to the operation of
surface forces. Adsorption can also occur at the interface of a
condensed and a gaseous phase.

Is the IUPAC definition completely general?  No.  It doesn't account for adsorption between supercritical fluids and solids. But it at least covers all the routine phase combinations.
As to the issue of whether IUPAC's defintion of solutions (which limits the solvent to be a condensed phase) contradicts its defintion of adsorption: While it could have been written more explicitly, a close read indicates there's not necessarily a contradiction.  Let's take each sentence in turn:
The first sentence concerns a liquid phase in contact with a solid or liquid surface.  Here the adsorbent is referred to as a &quot;dissolved substance&quot;, which would make it the solute within a liquid solution:

An increase in the concentration of a dissolved substance at the
interface of a condensed and a liquid phase due to the operation of
surface forces.

Note that the solute can be (prior to its dissolution) a solid, liqud, or gas.
The second sentence concerns a gaseous phase in contact with a solid or liqid surface.  Here no explicit mention is made of a dissolved substance:

Adsorption can also occur at the interface of a condensed and a
gaseous phase.

Thus we could parse IUPAC's language to mean that adsorption involves either the solute within a liquid solution that is in contact with a liquid or solid surface, or a gas that is in contact with a solid or liquid surface (where that gas could be pure, or part of a mixture).