An EV pack was equipment that Starfleet officers wore on training missions.

At one point during the Omega training mission on Titan in 2147, the EV pack that Commander Charles Tucker III was carrying froze up, causing him to develop nitrogen narcosis. In his narcosis induced delirium, Tucker attempted to remove his helmet. However, Archer was able to convince the commander to remain calm and keep his EV pack intact. (ENT: "Strange New World")

In reality, "EV" is a commonly used abbreviation of Extravehicular. Although there is no canon evidence to suggest the same is true of "EV pack" in the Star Trek universe, it seems likely.
The Star Trek Encyclopedia (4th ed., vol. 1, p. 252) described the "EV pack" as the life support module of a Starfleet EV suit.
Community content is available under CC-BY-NC unless otherwise noted.