Ok I need to step in and clarify what I thought should be obvious, and then maybe a little bit more. I guess to start off answering the OPs question. Yes. Delta 9 THC is the exact same thing as regular THC found in cannabis.
The Delta or Δ is referring to where the double bond is in the benzopyran ring. There are 6 possible deltas. Δ6, Δ7, Δ8, Δ9, Δ10, Δ6a-10a. It's a hexagon so there can only be 6 places on the ring that the double bond can sit at. There is something called Δ11 THC also known as exo THC but I don't like the terminology for reasons Ill give in the end.
As far as I know only Δ8 and Δ9 are really considered active.
Technically if you use the outdated monoterpenoid numbering system you can come up with a Δ5, but Δ5 in the monoterpenoid system corresponds to Δ7 in the dibenzopyran system. No researchers still use the monoterpenoid numbering system so let's not either, and keep the confusion to a minimum.
Ok now for why are these even called THC in the first place?
T = tetra = 4 in greek,
H = hydro= referring to the number of extra hydrogens
C = referring to Cannabinol also known as CBN
Now if you go look at the structure of CBN and you go back to the benzopyran ring you will see instead of 1 double bond in the hexagon you see 3. The ring is what we call aromatic. There are only 3 places that a hydrogen can fit. So there's 3 hydrogens. If you look at THC's (any of them) you'll see that they have 7 hydrogens. 7 - 3 = 4 = tetra. Tetrahydrocannabinol
The system applies to HHC as well, which has 9 hydrogens of the benzopyran ring. 9 - 3 = 6 = Hexa
Hexahydrocannabinol.
As far as Δ11/Exo-THC
https://en.wikipedia.org/wiki/Delta-11-Tetrahydrocannabinol
The double bond is moved outside of the ring and there are 8 hydrogens. 8 - 3 = 5 = penta. So it's probably more accurately described as a pentahydrocannabinol or PHC.
Most of this research was done a long time ago. There's really not too much new here besides people putting these cannabinoids in the bodies at larger scales than we've ever seen.