Things that are analog are assumed to have a continuous voltage measurement, meaning there can be any voltage level, not just high or low. In a circuit that ranges from 0-5V, an analog signal could be anything between those two levels, such as 2.3V or 4.7V. In reality, everything is analog, but to simplify things, in certain situations, we assume some signals can only be one of two levels, which we then call digital.
Characterized by a linear process in which a variable takes on a continuous set of values.
Electronic Devices : Conventional Current Version, 9th Edition by Thomas L. Floyd