what is considered spotting?

My period since I started birth control has been extremely light. A tampon is hardly necessary and I feel like I could almost get by with nothing. Should this be a concern and is it considered spotting or just a light flow?
Also because it is so light should I be worried about being pregnant or is this still considered an actual period? It has been getting lighter and lighter as the months go by but it is still on a normal schedule that follows my birth control.