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
Analogue electronics (American English: analog electronics) are electronic systems with a continuously variable signal, in contrast to digital electronics where signals usually take only two levels. The term "analogue" describes the proportional relationship between a signal and a voltage or current that represents the signal. The word analogue is derived from the Greek word ανάλογος (analogos) meaning "proportional".