Magnitude
The astronomer's brightness scale, and it runs backward: the lower the number, the brighter the object. Venus shines near -4, bright stars near 0, and past 6 the naked eye gives up.
Magnitude measures how bright something looks from Earth. The scale is ancient, from Hipparchus ranking stars first magnitude down to sixth, and the ranking stuck, so the numbers run the opposite way from instinct: brighter means lower. Each step of one magnitude is roughly two and a half times the brightness, and the brightest objects push below zero.
Some landmarks. The Sun sits near magnitude -27, the full Moon near -13, Venus at her best near -4.5. Jupiter and Mars can pass -2, Saturn hovers around 0 to 1, and Sirius, the brightest star, holds -1.4. Under a dark sky the naked eye reaches about magnitude 6; Uranus, near 5.7, is technically visible and famously ignored. Neptune and Pluto need optics.
Astrology borrows the number as a reality check rather than a doctrine. The magnitude beside a planet on the current planets table says whether the body under discussion is blazing over the rooftops or invisible without a telescope, which is worth knowing about a sky you are reading.