Overdub
Definition
The process of recording new material onto an existing multitrack recording, layering additional performances without replacing what was previously recorded. Overdubbing allows studio productions to be built incrementally, one element at a time.
In Simple Terms
Recording new material on top of what's already been recorded. The guitarist plays along to the existing drums and bass through headphones, adding their part without replacing anything. It's how songs are built layer by layer.
In Practice
After tracking a full band live, the producer has the lead guitarist overdub a refined solo in isolation, listening to the full band recording through headphones to perform against the complete arrangement.