Somnor Disc

Somnor discs were devices used by the Morpho to make humanoid minds receptive to their hypnotic control. The discs were placed on the First Doctor, Susan, Ian and Barbara on the first night of their visit to Morphoton. However, Barbara's disc slipped off and thus her mind was the only one unaffected when she awoke the next morning. The effect the discs had on the others was broken when the Morpho were killed. (TV: "The Velvet Web")

Behind the scenes
The name of the devices was never given on screen, only in the script (as revealed by the Info Text on the BBC DVD release).