The central computer that runs a starship's systems and answers spoken requests from the crew, most famously with the voice of Majel Barrett Roddenberry.
Every Starfleet ship has a main computer that stores its library, controls its systems and talks to the crew. On Kirk's Enterprise it runs on the duotronic circuits invented by Richard Daystrom and speaks in a flat mechanical voice.
In The Next Generation, Deep Space Nine and Voyager the computer is addressed simply as Computer, works through the LCARS interface and runs the holodecks and replicators. Majel Barrett Roddenberry voiced it across these series and several films. The USS Voyager adds bio-neural gel packs, which can fall ill, as in the episode Learning Curve.
Some computers become more than tools. In Discovery the ship's computer merges with the ancient Sphere data and emerges as Zora, a new life-form. In Starfleet Academy the computer of the USS Athena is voiced by Brit Marling.