A tank is a large powerful armoured fighting vehicle.
The phaser and disruptor tank is the most powerful fighting vehicle fielded by ground forces. These tanks usually have a forward energy weapon, one turreted weapon and sometimes missiles. (LUG RPG: The Dominion War Sourcebook: The Fires of Armageddon)
In 2410, Captain Kanril Eleya encountered a Terran Starfleet tank while engaging their ground forces during a mission into the mirror universe. The tank was impervious to phaser fire and the away team lacked anti-armor weapons, but Kanril was able to disable it by throwing a photon grenade down the hatch, killing the crew. (Bait and Switch: The Wrong Reflection)
Starfleet deployed a number of T-204 Hayes main battle tanks from the USS Bajor into the fighting on Kobali Prime. One was disabled, while another's crew was killed by an armor-piercing shell that penetrated the cabin. (Bait and Switch: "Last Rights")