SDI stands for Serial Digital Interface and SD stands for Standard Definition.
When we talk about an SD-SDI signal we talk about a standard digital video signal this means:
Format: 4: 3/720 pix. × 576i active lines / Binary Regime: 270 Mbps
Format: 16: 9/960 pix. × 576i active lines / Binary Regime: 360 Mbps
There are people who confuse and refer to the SDI signal when it means SD, this can lead to error since the SDI signal can be SD, HD, 3G, 6G or 12G.