Apabhraṃśa is a term used by Sanskrit grammarians since Patañjali to refer to dialects of North India that deviate from the norm of Sanskrit grammar. The term apabhraṃśa in Sanskrit literally means "corrupt" or "non-grammatical language".
It is used as a cover term for the dialects forming the transition between the late Middle Indic and early Modern Indic languages (e.g. Brij Bhasha), spanning the period between the 6th and the 13th centuries, though some scholars use it more narrowly to refer primarily to the transition period, leaving the earlier part to the Middle Indo-Aryan languages.
The term Prakrit (which includes Pĝli) is used for the popular dialects of India which were spoken until the 4th - 8th century, but some scholars use the term Prakrit throughout the Middle Indo-Aryan period.
Middle Indo-Aryan languages gradually transformed into Apabhraṃśas which were used until about the 15th century. Apabhraṃśas evolved into modern languages which are equally today spoken by millions of people.
Languages such as Hindi (337 million speakers), Bengali (232 million speakers), Marathi (90 million), Urdu (160 million speakers), Gujarati (46 million speakers), Sinhala (15 million speakers) are all representative languages of large modern day states, unlike Sanskrit (>50 thousand speakers) which has fallen out of modern day use.
The boundaries of these periods are somewhat hazy, not strictly chronological. The modern north Indian languages are often considered to have begun to develop a distinct identity around the 11th century, while the Apabhraṃśas were still in use, and became fully distinct by the end of the 12th century.
- See Wikipedia article on Apabhramsha for more information