What are milestones?
Skills such as taking a first step, smiling for the first time, and waving "bye-bye" are called developmental milestones. Children reach milestones in how they play, learn, speak, act and move such as crawling, walking etc.
The Center for Disease Control has a milestone checklist and questionnaire. If you feel for any reason that your child is not hitting their milestones talk to your pediatrician.
Center for Disease Control