There are 2 basic types of deciduous tree leaves, simple and compound. Simple leaves have a single leaf blade per petiole (a leaf stem). Some common examples include Oaks, Maples, Cherries and Poplars.
Compound leaves have multiple leaflets per petiole, therefor all of the leaflets together create a single leaf. Some common examples include Black Walnut, Locust and Ash.
So now, if someone refers to a leaflet as a leaf, you can reply “excuse me, but that’s not a leaf, it’s a leaflet“.