The term 'die out' is a phrase used to describe the process of a species or population becoming extinct. The crossword solutions for this term are PETER (5 letters), CEASE (5 letters), and PERISH (6 letters). Peter is a biblical figure who is known for his faithfulness and martyrdom. CEASE is a verb meaning to stop or discontinue an action. PERISH is a verb meaning to die or be destroyed. All three words can be used to describe the process of a species or population dying out.