To depart and forget, or neglect to bring or take along: She wept thinking she'd been left behind.

1. When you leave someone or something behind, you go to a different place.
2. When you leave something or someone behind, you do not take them with you because you forgot or because you cannot or do not want to take them.
3. When you move, learn, or work faster than others in your group, you leave them behind.