There has been much debate about what constitutes a sign. Charles Sanders Pierce suggested a typology which still serves as the basis for much of the research in the field.
Icon When referring to computer interfaces, the term icon is everywhere.
However, the icons that you and I refer to may not be what Pierce had in mind. Take the control panel icon. According to Pierce, it should have a similarity to the thing referred to. It doesn't, and is really, in Pierce's schema, a symbol. Each indicates position or proximity by virtue of its pointing.