Originally, Godhead was just a variant spelling and pronunciation of Godhood, and so carried the exact same meaning as that word. But that's not how it's usually used today.
I've seen people who agree with he doctrine of The Trinity as it's usually defined, but don't like the word "Trinity" for whatever reason, say we should say The Godhead instead. And even others not suggesting we use it as a synonym for Trinity, still seem to mean something similar by it.
Let's look at all three verses it is used in and their corresponding Greek words.
"Forasmuch then as we are the offspring of God, we ought not to think that the Godhead is like unto gold, or silver, or stone, graven by art and man's device."Godhead here is Theion, Strong Number 2304. This is the only of the three words used more then once, though the others is a slightly different form. Theios in 2 Peters 1 verses 3 and 4, there it is translated "divine". And I think likewise "the divine" is how it should be rendered in Acts 17.
"For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; so that they are without excuse:"Here it is Theiotes, Strong Number 2305. Here I agree with the Strongs that it should be rendered "Divinity". And so it would be here that the "Godhood" meaning would be accurate.
"For in him dwelleth all the fulness of the Godhead bodily."Here the Greek word is Theotes, Strong Number 2320. This word is a bit more mysterious. One website says it also means divinity but with "abstractly" in parentheses. I've also seen it defined as "The substance of being God".
In this case it is useful to talk about some of the Greek words around it.
The word for "fulness" is Pleroma. Because the Gnostics used Pleroma a certain way, it suits some people to say Paul is using it that way here. But this is the same word for "fulness" used when he refers to the "fulness of the gentiles" in Romans 11:25. It just means fullness.
The word translated "Bodily" is Somatikos Strong Number 4985. It's a variation of Strong Number 4984. It could also equally accurately be translated Corporeally or Physically.
The term Homoousion "of the same substance" was a key focus of controversy at the First Council of Nicea in defining The Trinity. While I can sympathies with those who objected to the word because it was not used in The Bible and seems to have been coined by Gnostics. The point of the Nicean Homoousion doctrine is supported by Colossians 2:9.
If I were to make my own translation, Colossians 2:9 might likely be the only place where "The Godhead" is used.