A type of camera sensor notable for excellent low-light performance.
A camera sensor is like a computer chip. A traditional "front-side illuminated" sensor is manufactured with a base layer on the bottom, then the light-sensitive layer, then the electrical wiring layer on top. In the complete camera module, a lens focuses light on the top of the chip.
However, the top wiring layer blocks some of the light, hurting performance.
In a back-side illuminated sensor, the chip is made much the same way, but a last step is added where the chip is flipped over and the base layer shaved off (or similar process.) This allows light to reach the light-sensitive layer without wiring in the way, improving performance, especially in low light.