What is Glacial period; interglacial; glacial cycle?

A glacial period is a cold interval within an ice age, during which the ice sheets grow far beyond their present size. An interglacial is a warm interval between two glacial periods, with ice sheets close to today’s or smaller; we live in one, which began about $11\,000$ years ago. A glacial cycle is one glacial period with the interglacial that ends it.