Borg adaptation
Adaptations are a variety of mechanisms by which the Borg defend themselves from attack.
Shields
Adaptation is generally used in the context of shields, in particular, Borg cube shields. In The Wrath of Khan Spock performs what the Borg would call adaptation, by matching the Enterprise's phaser frequency with the shield frequency of the Reliant.
The Borg match frequencies with incoming frequency-based weapons. At Federation power levels, this makes Borg cubes relatively secure from Federation starships. At higher power levels or against weapons with no frequency such as Species 8472's weapons, adaptation provides mediocre defense.
Frequencies
The most common meaning of adaptation in Star Trek is with frequency based weapons. Frequency based weapons such as phasers become ineffective when the Borg have adapted. In combat, Federation starships rapidly change the frequency of their weapons, to hinder adaptation. However, rapidly rotating frequencies is, at best, a delaying tactic. Federation hand phasers can also be programmed to rapidly change frequencies, allowing a security officer to disable or kill several drones before the Borg adapt and render his weapon useless.
Repair
Adaptation appears in The Next Generation "Q Who" as a formidable self-repair mechanism. After hit by a photon torpedo, the Borg cube repairs itself. The Borg cube's outer hull is constructed of ductile material conductive to self-repair. It is unclear whether all Borg cubes possess this ability. Other Borg cubes have never shown this ability for rapid repair.
Projectile Weapons
In Star Trek: First Contact, Captain Picard fires a Thompson submachine gun at two Borg drones. The drones do not adapt and are killed. The Borg possibly "adapt" to projectile-weapon species with armored tactical drones and tactical cubes, seen in Voyager "Dark Frontier" and Voyager "Unimatrix Zero." However, no evidence indicates Borg drone shields are effective against kinetic penetrators.