As a rule, anything that a plant uses to make baby plants ( seeds) is a fruit, and any other part of the plant that is hacked off to be eaten, is a vegetable. So cucumbers, squash, tomatos and pumpkins are all considered fruit. Lemons are the reproductive structures of lemon trees, and are there for fruits, in the citrus family along with oranges, limes, and tangerines(cuties).
Seedless fruit has been bred to be that way, but is still fruit.
Lemons aren't used in salads(fruit or otherwise) because they have a seriously tart and sour flavor, ( you probaby knew that) so you won't find chunks of super sour lemon in anything, but it is used in some fruit saladd to kepp the bananas and apples from turning brown, and it is a common component of salad dressings because it has a fresh taste, that when mixed with othere flavors, is pretty tasty, but they are fruit, just too sour to be part of fruit salad or regular salad in any obvious forms.