The gradual exposition of Raag emphasizing Vaadi, Samvaadi and other salient features of the raag in a slow
tempo is known as Alaap.
The word alaap means a dialog or conversation. Alaap is a dialog between the musician and the raag. Alaap reflects the depth, the temperament, creativity and training of the musician.
In alaap, the musician improvises each note gradually. Beginning with the lower octave and in a slow tempo and techniques like kana swar and meend etc. The alaap is sung in the beginning of the raag at the time of a performance. This is also known as the Vistaar. When the musician starts rendering a Bada Khayal / Chotaa Khayaal (bandish) the tabla or any other percussion instrument joins. Alaap is used again with the composition, this time with the rhythm as well. This alaap is slightly faster and and rhythmic.
Some times the words of the song are also improvised with notes. This is known as Bol Alaap.
Alaap is usually sung in Aakaar i.e. without pronouncing any syllables only using the sound "aa" of the vowel. Sometimes syllables like teri, Nom, Tom are also used for singing the alaap this type of singing is called Dhrupad and Dhamaar. It is said that Tansen used to sing in nom, tom and Amir Khusro for the first time introduced khayal gayaki in Indian Classical Music.