What does BC<number> stand for in image formats?

I was scrolling trough the formats and I saw this BC part. To be specific I saw it in VK_FORMAT_BC1_RGB_UNORM_BLOCK so the “BC1” part.
From descriptions I am guessing it has to do with compression but I can’t figure out what it is supposed to mean or stand for.

I don’t want to look like a jerk, but…
https://www.google.ru/search?newwindow=1&ei=HCt8WsudBoP-swG5_Zf4Bg&q=bc1+texture&oq=bc1+texture&gs_l=psy-ab.3..0j0i22i30k1l2.44782.44931.0.45136.2.2.0.0.0.0.142.248.0j2.2.0…0…1c.1.64.psy-ab…0.2.247…0.6-6sAGd8u60

Ooh… lol thanks… sometimes my brain refuses to think logically. xD

While being jerks, there’s also a manual:
https://www.khronos.org/registry/vulkan/specs/1.0/html/vkspec.html#appendix-compressedtex-bc
https://www.khronos.org/registry/DataFormat/specs/1.2/dataformat.1.2.html#S3TC