*spek-

The core idea of the root was:

observing

It was a pretty big family in Latin and many of these words have made there way into English. The most obvious group is the one around spec(t) 

• inspect(ion) (“look into”)
• aspect (“part you see”)
• introspect(ion) (“look inward”)
• expect, expectation (“looking outward”)
• perspective (“looking beyond”)
• prospect (“looking ahead”)
• suspect, suspicious (“looking under”)
• retrospect (“looking back”)
• respect (“look back at someone”)
• speculate (“to take perspectives”)

Another group is the one around scope.

• the scope (“what view”)
• telescope (“view large”)
• microscope (“view small”)
• gyroscope (“watch a circle”)
• horoscope (“watch the hour”)

And there are still more words to be found like spy and espionage and also species (“what you can see”) and its relative special, which originally was “the certain type“.