oops sorry didnt mean to downvote your answer, but it is a 3.5 mm headphone jack, though now that you point out a difference in the tips of the headphones mine has 3 layers and the splitter has only 2.
My computer has two jacks, one for microphone and one for headphones. I have a headset with a detachable gaming mic and the headset only has one cord. Now I bought a splitter thinking that would work and I would be able to use the microphone after that. Now since I'm a total dunce when it comes...