1. Relating to a system or device that represents data variation by a measurable physical quality.
2. Measuring or representing data by means of one or more physical properties which can express any value along a continuous scale; for example, the position of the hands of a clock is an analog representation of time.
3. An organ or structure that is similar in function to one in another kind of organism, but which is of a dissimilar evolutionary origin; such as, wings of birds and the wings of insects are analogs.
4. A chemical compound that has a similar structure and similar chemical properties to those of another compound, but which differs from it by a single element or group.

The antibiotic amoxicillin, for example, is an analog of penicillin, differing from the latter by the addition of an amino group.