Editorial
Discovery
Also: disclosure
Definition
The pre-trial process by which parties obtain evidence from one another, through document requests, interrogatories, depositions and requests for admission.
In plain English: The stage before trial where each side has to hand over relevant documents and answer questions.
You did not name a jurisdiction, so this shows the US definition. The term is defined differently elsewhere — see below.
Usage
England calls the equivalent process disclosure, and its scope is narrower than US discovery. The two are not interchangeable.