Tempo — the speed at which matras pass. The broad bands are vilambit (slow), madhya (medium), and drut (fast), with ati-vilambit and ati-drut at the extremes. By extension the word also covers a player's command of time itself, as in the compliment "strong laya."
Laya is held steady while density changes around it; rushing when the bols get thick is the classic fault. The related term layakari means deliberately playing rates against the pulse — that is where dugun, aad, and kuad live.
Reference: Martin Clayton, Time in Indian Music: Rhythm, Metre, and Form in North Indian Rag Performance (Oxford University Press, 2000).