1. Any of the geologic periods that embraced an ice age. For example, the Quaternary period may be called a “glacial period. ” 2. Generally, an interval of geologic time that was marked by a major equatorward advance of ice. This may be applied to an entire ice age or (rarely) to the individual glacier “stages” that make up an ice age. The term “period” here is not used in the most technical sense of a geologic period.