SDI stands for Serial Digital Interface and SD stands for Standard Definition.

When we talk about an SD-SDI signal, we are referring to a standard digital video signal. This means::

Format: 4:3 / 720 pix. × 576i active lines / Bit rate: 270 Mbps
Format: 16:9 / 960 pix. × 576i active lines / Bit rate: 360 Mbps

Some people confuse the SDI signal with SD, which can lead to errors since the SDI signal can be SD, HD, 3G, 6G, or 12G.