A crossover is, in short, a story that involves more than one title; that is, a story that isn't contained within a single series, miniseries, or one-shot story.
Crossovers can be small (The Missing HQ Caper involved two titles and two Writers) or large (Retcon Hour had fourteen writers over a massive patchwork of stories). Crossovers can involve two characters from the same universe, or massive numbers of characters and concepts from completely different Imprints.
RACCies:Favorite Crossover was awarded to the RACC readers' favorite crossover of the year until it was incorporated into RACCies:Favorite Arc.
See Category:Crossovers for a full list of crossovers involving the LNH. See also The CrossOver Caper, the Crossover Queen, the Crossover Gem, the Crossover Citadel, and the Ultimate Crossover.