The act or process of joining or the condition of being joined. A place where two things join or meet, especially a place where two roads or railway routes come together and one terminates. A transition layer or boundary between two different materials or between physically different regions in a single material, especially: A connection between conductors or sections of a transmission line. The interface between two different semiconductor regions in a semiconductor device. A mechanical or alloyed contact between different metals or other materials, as in a thermocouple.