A lek (from Swedish lek, a noun which typically denotes pleasurable and less rule-bound games and activities) is a tournament (the males of certain species of animals for the purposes of competitive mating display), held before and during the breeding season, day after day, when the same group of males meet at a traditional place and take up the same individual positions on an arena, each occupying and defending a small territory or court. Intermittently or continuously they spar with their neighbours one at a time, or display magnificent plumage, or vocal powers, or bizarre gymnastics...

Though they have territories, yet they have a hierarchy with the top-ranking males typically placed in the middle and ungraded lesser aspirants ranged outside. Females come to these arenas in due course to be fertilized, and normally they make their way through to one or other of the dominants in the centre.

The term was originally used most commonly for Black Grouse (orrlek) and for Capercaillie (tjäderlek), and lekking behaviour is quite common in birds of this type, such as Sage Grouse. However it is also shown by birds of other families, such as the Ruff, Great Snipe, Musk Duck Green Hermit and Kakapo, by some mammals such as the Uganda kob (a grazing antelope) and by some species of fish and even insects like the midge and the Ghost Moth. The rut of deer is also very similar. There is some dispute among ethologists as to whether the lekking behaviour shown by animals of widely different groups should really be treated as the same, and in particular whether similar selective pressures have led to their emergence.