Overtired or undertired

What is the.difference between overtired and undertired? Our little one is 6.5month. He wakes around 7am and goes to sleep around 8pm (goes down independently, sometimes a quick pat on the chest but can settle self. Also dream feed between 10pm-10.30pm) He has between 2.5-3hrs day sleep over 3 naps. Yesterday he had 2.5 hrs sleep and was awake for over an hr. Recently he is waking more at night. I know babys wake but he seems alert, hard to get back to sleep and can be awake for a while. We are trying to figure out whether hes overtired or undertired. His sleep environment is really good, we thought he could be teething but he doesnt seem in pain and his temperature is fine. Any help would be appreciated.