Standard Definition refers to video, or sometimes audio, encoded for use with legacy systems like analog TV and CD Audio (CD-DA).
In the case of video it refers to SDTV resolutions, as defined in the ITU-R BT.601 specification. It may also refer to the actual analog video signals associated with digital SDTV. Widescreen video encoded anamorphically for DVD is also Standard Definition as its "sqeezed" into a Standard Definition Frame. Besides DVD, other video sources considered Standard Definition are VCD, SVCD, DV (from SD camcorders), and the various non-standard resolutions used for MPEG-4 ASP (DivX, XviD, 3ivX, etc, ...) encoding. Digital TV may also be broadcast in Standard Definition as the standard broadcast formats - ATSC and DVB, allow for both Standard Definition and High Definition.
For audio the definition is somewhat less clear, although it generally refers to audio with a Samplerate no higher than 48kHz (48,000 samples per second) and sample depth of no more than 16 bits. This includes Audio CDs and the standard audio used for DVD-Video.