3.5mm headphones

can you use regular 3.5mm headphones with iMacs?

Yes. The audio out socket is 3.5 mm.

Apple - iMac - Tech Specs

why do they call it a minijack??

To my knowledge there are 4 sizes of audio jack of the kind used on headphones - 2.5 mm, 3.5 mm, 6.35 mm and 6.5 mm. The 'mini' name was to distinguish between the larger jacks, which were commonly used for some years, and were too large for tight situations, and the next size down, the 3.5 mm. But it's got slightly complicated by the arrival of the 2.5 mm jack, but its use is not as common as the 3.5 mm. There are adaptors to change jack sizes.