Pentium is Intel’s brand name for a family of computer processors — the chip that does the thinking in a PC. In the 1990s and early 2000s, if you had a personal computer, there was a fair chance a Pentium was inside it. Older learner dictionaries still say Pentium chips are in most of the world’s PCs; though actually, that was true of that era, not of now. Intel moved on to names like Core i3, i5, and i7, and Pentium later became a cheaper line. You will still see the name on old machines, in old manuals, and in jokes about “a Pentium” meaning a computer from that time. It is a trademark, so it is capitalised. People used to tell the types apart by speed (a 133 MHz Pentium, a Pentium 4). It is not a general word for any processor, and it is not a word for the whole computer.