asthai - ਅਸਥਾਈ - In Keertan, we find one line keeps getting repeated time and again. This common line, which is repeated each time after a different line, is known as asthai in Indian classical music, or Chorus in the western music.
antra - ਅੰਤਰ - In Keertan, or any music, after singing the asthai, the repetitive line, a different line is sung and upon its completion the asthai is sung again and then a second different line is sung, followed by asthai. This changing line is known as antra in Indian classical music.