What does "week" mean?

Week: Note: Although it [the week] did not enter into the calendar of the Greeks, and was not introduced at Rome till after the reign of Theodesius, it has been employed from time immemorial in almost all Eastern countries. Encyc. Brit. Feast of Weeks. See Pentecost, 1. -- Prophetic week, a week of years, or seven years. Dan. ix.