On the third Wednesday of every month, a bar and grill has a "backgammon night." The bar provides backgammon boards and pieces, and a fairly regular group of players come in, share a few drinks, and play backgammon together. What is this an example of?

A lifestyle enclave.

Explanation:

A lifestyle enclave is a term first coined by Robert Neelly Bellah. Robert was an American sociologist. In his book "Habits of the Heart" Robert first used this term.

A Lifestyle enclave is a feature shared by people who have some common private life. People of the lifestyle enclave group reveals their identity through patterns of leisure activities, appearance, etc. that are common.

In the given question, those people who share the quality of playing backgammon and drinking together is an example of "lifestyle enclave" according to the definition provided.