Q: How do I respond to 'What is X?' questions?
A: If the question has a clear, single answer, then it is valid. For example: "What is a left coset?" is a "What is X?" question, but there is implied context.
If the question lies without context whatsoever, leave a comment asking for more details about the asker's background, where they saw the term, etc.
If the question has only subjective answers, [down vote?] / [vote to close?], and request the asker to more carefully consider what they intend to learn from replies to their question - and reword the question, making it clear what they expect in reply.
Related: http://meta.math.stackexchange.com/questions/168/are-what-is-x-questions-acceptable
