A witness’s out-of-court testimony that is reduced to writing (usually by a court reporter) for later use in court or for discovery purposes.

In Plain English:

A deposition is when the attorney asks questions of a party from the opposite side of the lawsuit, or of a witness. The attorney for the person answering questions is also there to help by objecting and instructing the person answering whether or not they have to answer certain questions. A court reporter will be present to create a transcript of the deposition. The testimony given at deposition can be used later during the trial, if there is one.