The process of laying new audio material in, over, or with existing material. Generally this applies to adding parts to a multitrack recording. You could have the basic tracks of a band down and then add vocals or other instruments. These are known as overdubs. The word originates from the word dub. Years ago, when all we had were single track tape machines, the only way to add material to an existing recording was to dub that tape to a second machine and mix in the new material along the way. This process was known as overdubbing.