Lineup lock typically refers to a set time at which all fantasy lineups in a given league or contest must be submitted. In season-long fantasy basketball leagues, lineup lock can occur on a daily or weekly basis.
In leagues with daily lineups, lineup lock usually occurs at the time the first game of a given night begins. In leagues with weekly lineups, lineup lock is set for the same time each week. Once that time passes, owners are not allowed to make any lineup changes.
Lineup lock is also utilized in daily fantasy basketball (DFS). In this context, it refers to the time at which a contest begins and no more lineup changes or swaps can be made.