CC Madhya 20.263

sṛṣṭi-hetu yei mūrti prapañce avatare
sei īśvara-mūrti ‘avatāra’ nāma dhare
Word for word: 
sṛṣṭi-hetu — for the purpose of creation; yei mūrti — which form of the Lord; prapañce — in the material world; avatare — descends; sei — that; īśvara-mūrti — form of the Lord; avatāra — incarnation; nāma dhare — takes the name.
Translation: 
“The form of the Lord that descends into the material world to create is called an avatāra, or incarnation.