An activation sequence was a sequence sent to activate a computer program.
In 2152, Commander Trip Tucker asked Doctor Phlox if he had the activation sequence for the repaired neutron microscope. (ENT: "Stigma")
In 2153, the Borg sent some kind of activation sequence, a transmission, to activate the modifications to the warp plasma regulators they made aboard Enterprise NX-01. (ENT: "Regeneration")
In the holoprogram The Voyager Encounter, the holographic duplicate of Captain Janeway initiated the Borg activation sequence, a security program sending Borg drones to eliminate invaders. (VOY: "Living Witness")
In an attempt to convince the Overlooker to dismiss his attack plans for the USS Voyager, The Doctor told him that the starship had a photonic cannon. When the Overlooker responded that his sensors are not displaying an activation sequence, The Doctor told him that this weapon was impervious to sensors. (VOY: "Tinker Tenor Doctor Spy")