1. Physics. Information in a continuously variable form, such as analogue data are represented by an infinite number of shades of grey, instead of black and white. The opposite is digital.
2. Biology. Organs which differ in structure or development but are similar in function, e.g. gills and lung.
3. Chemistry. A chemical compound is called analogue if it is structurally similar to another but is a bit different from it by a single element.