Standard-definition television (SDTV) is a television system that uses a resolution that is not considered to be either enhanced-definition television (EDTV) or high-definition television (HDTV). The term is usually used in reference to digital television, in particular when broadcasting at the same (or similar) resolution as analog systems. The two common SDTV signal types are 576i, derived from the European-developed PAL and SECAM systems with 576 interlaced lines of resolution; and 480i, based on the American National Television System Committee NTSC system.

In the US, digital SDTV is broadcast in the same 4:3 aspect ratio as NTSC signals. However, in areas that used the PAL or SECAM analog standards, standard-definition television is now usually shown with a 16:9 aspect ratio, with the transition occurring between the mid-1990s and mid-2000s. Older programs with a 4:3 aspect ratio are shown in 4:3.

Standards that support digital SDTV broadcast include DVB, ATSC and ISDB. The last two were originally developed for HDTV, but are more often used for their ability to deliver multiple SD video and audio streams via multiplexing, than for using the entire bitstream for one HD channel.^{[clarification needed]}