A measure of how reflective a surface is.
An albedo of 0 would indicate a surface that absorbs all light that is shined upon it. An albedo of 1 would indicate a surface that reflects all light that is shined upon it. The albedo of fresh snow is around 0.8. The albedo of a road surface is about 0.1.
Albedo is not the same as darkness. The Moon has a relatively low albedo (0.12) but it looks bright in the sky for two reasons. First, it receives a huge amount of light from the Sun. Although it only reflects a small proportion of this light, that’s still an awful lot of light being reflected. Second, it’s very close to the Earth, so we receive a relatively high proportion of the Moon’s reflected light. Venus has a much higher albedo (0.65) and it is closer to the Sun than the Moon is, so it is receiving and reflecting a lot more light than the Moon. But Venus is much further away from the Earth, so a far smaller proportion of this reflected light reaches the Earth. Nonetheless, we get enough of this reflected light to make Venus the third brightest object in the sky, after the Sun and the Moon.