A slut is commonly defined as a woman who has casual sex with multiple men. However there are different definitions.
The theoretical slut
In the purest sense, a slut is a woman who just wants lots of sex - she does not care who she has sex with. However, this type of woman does not exist. Most women referred to as "sluts" have sex with many men, but the men they have sex with are almost always attractive.
The common "slut"
Most women referred to as "sluts" have sex with only attractive men. These "common sluts" are girls ranging from 4-7 in attractiveness, and take turns getting fucked by multiple 8+/10 males. By doing this they boost hypergamy, leaving their looksmatches incel.