A data buffer was a piece of communications technology.
In 2151, Captain Jonathan Archer told Ensign Travis Mayweather to see if the data buffer of the communications tower at the Terra Nova colony was still intact. Later, he told Sub-commander T'Pol and Ensign Hoshi Sato to work on the data buffer and retrieve any transmissions sent by checking the transmission logs. (ENT: "Terra Nova")
The scene in which Mayweather inspects the communications tower was planned to be filmed but not part of the final episode. In this scene, according to the props section of the call sheet, a prop data buffer was seen.
In 2376, Captain Kathryn Janeway noticed that Commander Chakotay activated the data buffer when the two entered cargo bay two. (VOY: "The Voyager Conspiracy")