Tissue Compression Eliminator
The Tissue Compression Eliminator, also known as the TCE, was a trademark weapon of the Master, especially so following his assumption of the appearance of the Trakenite Tremas. (DW: The Keeper of Traken)
As its name suggests, the TCE killed by drastically compressing the bodies of its targets to the point where life functions ceased. The Master liked to leave the shrunken doll-like corpses of his victims as a calling-card for the Doctor to discover.
The Master later attempted to improve the design of his weapon, but a laboratory accident resulted in both him and the aforementioned laboratory being reduced in size instead. This process did not appear to physically harm him. (DW: Planet of Fire)
The upgrade to the TCE later seemed to be perfected as on subsequent appearances targets appeared to be utterly disintegrated rather than simply killed and reduced in size.
The TCE, despite its name, was equally capable of shrinking inorganic tissue: victims' clothes were also shrunk, and on one occasion the Master demonstrated the weapon by shrinking unoccupied space-suits. The Doctor once described the method by which the Master killed as Matter Compression, which seems slightly more accurate. However, one must always bear in mind that the Master is an insane evil genius and one must not expect everything he says and does to conform to the highest standards of rationality.