Matter condensation was a miniaturisation technique used by the Master to kill a living being by shrinking it.
According to the Fourth Doctor, his nemesis had picked up the technique "somewhere on his travels", and this way was a "nasty sort of death". (TV: The Deadly Assassin)
The Master employed a Tissue Compression Eliminator to achieve the process. He used the weapon frequently over the years. (TV: Terror of the Autons et al.)