"Buy-In" is an overloaded term.
In the poker tournament world, the term "Buy-In" typically refers to the cost to enter a tournament. Some examples:
In Dr. Neau's Tournament Manager, the word "buy-in" represents something very specific. It is an instance of a player "buying in". A tournament managed in the application can actually define multiple options by which people can buy into the tournament...these are called "buy-in options"...and when a player execises one of those options that creates a "Buy-In" record.
Sorry for the confusion...it really doesn't matter, but you'll see references to each so I wanted to make sure you understood the reasoning behind it. I also apologize for the inconsistent use of the hyphen between "Buy" and "In".