In the world of microelectronics companies, STM stands for STMicroelectronics.

Formerly known as "SGS-Thomson", they are a French-Italian multinational company who are the number 1 supplier of MPEG decoder chips for set-top boxes, amongst many other things. Their huge product portfolio includes discrete components, ASSP's for a wide range of applications and a family of microcontrollers and associated IP.

As a footnote: They also bought Inmos, a semiconductor company set up by the British government in the late 70's and, rather short-sightedly IMO, sold a few years later.