I heard this on an HGTV television program last night.

Problem:
The HGTV program host used an ambiguous modifier.

Explanation:
An “entertaining room” could be a room for entertaining or a room that is entertaining.

Based on the context, I know that the program host meant the former, not the latter.

In contrast, an “entertainment room” can only be a room for entertainment.

I believe that the choice of “entertaining” instead of “entertainment” supports my “Devolution toward Simpler” hypothesis. It’s slightly simpler to say “entertaining” than it is to say “entertainment”. Each of these words has four syllables, but the “ing” sound is simpler to say than is the “ent” sound because the g can be swallowed whereas the t must be enunciated for clarity. Also, there is an intervening “m” in “entertainment” that does not occur in “entertaining”.

Once again, here is my unorthodox but fairly reliable method to determine which of “entertaining room” or “entertainment room” is the correct form: search Google separately for each of “entertaining room” and “entertainment room” with the quotation marks included in each search; the one with the dominant number of hits or matches is very likely the correct form.

For example, I just searched Google for “entertaining room” and got about 18,500 matches; I searched for “entertainment room” and got about 1,040,000 matches. The 56:1 dominance of “entertainment room” over “entertaining room” is a very good indicator that the former is the correct form.

Solution:
“Entertainment Room”