Alap - aspect of classical Indian music

"The Alap is the opening section of a typical North Indian classical performance. It is a form of melodic improvisation that introduces and develops a raga. In dhrupad singing the alap is unmetered, improvised and unaccompanied, and started at a slow tempo. For people unfamiliar with the raga form, it introduces the mode to the listener. It defines the raga, its mood, and the emphasized notes and notes with a secondary role. It's like an invocation. - Wikipedia"