To testify means to give evidence or make a formal statement about what you know, especially in a courtroom. When someone testifies in a trial, they sit in the witness stand, swear to tell the truth, and answer questions about what they saw, heard, or experienced. A witness might testify about seeing a car run a red light, or an expert might testify about scientific evidence.

The word comes from the same root as “testament,” which means a statement of what's true. When you testify, you're declaring something based on your own knowledge or experience. The act of testifying carries serious weight because people rely on your honesty to make important decisions.

Outside courtrooms, people use testify more broadly to mean speaking up about what they know to be true. A friend might testify to your good character when vouching for you. Someone might say “I can testify to how hard she worked” when confirming another person's effort. In religious settings, people testify when they share their beliefs or spiritual experiences with others.

The related word testimony refers to the statement itself: a witness gives testimony during a trial, or someone writes testimony about events they witnessed. Whether in court or in daily life, testifying means standing up and saying what you know, putting your credibility on the line for the truth.