A signal is basically an electrical or electromagnetic current that is used for carrying data from one device or network to another. It is a physical quantity. It varies with some dependent or independent variables.
So term of Signal can be defines as "A physical quantity which contain some information and which is function of one or more independent variables."
A signal can be analog type or digital type. Signal basically one dimensional and two dimensional. In one dimensional signal the function depends on a signal variable, i.e. speech signal whose amplitude varies with time while in multi dimensional signal depends on two or more variables, i.e an image because it is horizontal and vertical co-ordinates.