What is a crush?

The other day Will came home and asked Ann what does crush mean? She said it meant to smash something. He looked at her rather confused and said "no what does it mean to crush someone?" Ann then realized what he was talking about and said that is when someone likes someone else. So then Will told her that Anna told him that Kelsey crushed him! Will and Kelsey have had a real on and off relationship this year and it appears that they are on right now. Sometimes Kelsey just isn't in to him and so then he and Sadie start hanging out more, but right now it appears that Kelsey and Will crush each other!