Sometimes, two tags can mean the same thing. For example, girl and female have identical meanings. It makes sense that if you search for one, you should also get the results for the other.

e621 tries to fix this issue by using tag aliases. You can alias one or more tags to one reference tag. For example, if we aliased girl to female, then whenever someone searched for girl or tagged a post with girl, it would be internally replaced with female.

When a tag is aliased to another tag, that means that the two tags are equivalent. You would not generally alias rectangle to square, for example, because while all squares are rectangles, not all rectangles are squares. To model this sort of relationship, you would need to use implications.

While anyone can suggest an alias, only an administrator can approve it.