In the wider world, an almanac is an annual publication dedicated to information such as weather forecasts, tide tables, lunar cycles etc. A typical almanac will contain tabular information covering a particular field or fields, and will be arranged according to the calendar.
However, in the world of satellite navigation systems, the almanac is a regularly updated digital schedule of satellite orbital parameters for use by GNSS receivers.
The almanac for any given GNSS consists of coarse orbit and status information covering every satellite in the constellation, the relevant ionospheric model and time-related information. For example, the GPS almanac provides the necessary correction factor to relate GPS time to co-ordinated universal time (UTC).
The major role of the almanac is to help a GNSS receiver to acquire satellite signals from a cold or warm start by providing data on which satellites will be visible at any given time, together with their approximate positions. An ephemeris message is still required from each satellite for the receiver to compute the exact position, but it is the almanac for the constellation that gives the receiver its starting point.
The ionospheric model contained within the almanac is essential for single-frequency receivers to correct for ionospheric errors - the largest error source for GPS receivers. However, modern dual-frequency receivers have no need for this data as the dual-frequency design can correct for such errors without any assumed model.