Definition
Declaring mahjong is announcing that your fourteen tiles exactly match a line on the current card, which ends the hand immediately.
A mahjong call beats any other call for the same tile. Declaring in error makes your hand dead.
Read the full guide: American Mahjong Rules: The Complete Plain-English Reference.
See it on real tiles: the coach reads any hand and names its direction, free, at the free read. Members get the coach on call for their own games.
More terms: Pung, Kong, Quint, Pair, Charleston, Blind pass, Courtesy pass, Joker.