TRANSISTOR CONFIGURATIONS:

Before a transistor can be used, it must be connected into an input circuit (by two wires) and an output circuit (two wires).  However, because the transistor has only three terminals, one of the terminals must be in both the input and output circuits; this is then called "The Common terminal".  Three circuit configurations are possible and are illustrated in figure.

Note that the word 'common' refers to the transistor component connected to both the INPUT and OUTPUT.  In the common emitter configuration for example, the emitter is connected to both the input and output. 

Table shows the comparisons of the three transistor configurations