Describing an optical wave-guide or fiber that is designed to propagate light of only a single wavelength (typically 5-10 microns in diameter).
Single Mode fiber is a bit like a straw, you can only drink out of one end at a time. Single Mode is usually used to send signals over long distances. Multi Mode is the type that goes to rooms on campus.
Containing only one mode. When dealing with lasers, be aware of ambiguities because of the difference between transverse and longitudinal modes. A laser operating in a single transverse mode typically does not operate in a single longitudinal mode.