1. The natural renewal of a structure, as of a lost tissue or part. (biology-online.org)
2. Growth anew of lost tissue or destroyed parts or organs. (wordnetweb.princeton.edu)
3. In biology, the process by which some organisms replace or restore lost or amputated body parts. (global.britannica.com)
4. The action or process of regenerating or being regenerated, in particular the formation of new animal or plant tissue. (Google Dictionary)
Word origin: from Late Latin regenerationem (nominative regeneratio) “a being born again,” noun of action from past participle stem of Latin regenerare “make over, generate again,” from re- “again” + generare “to produce”.