A PhD is a university degree of a very high level. You do not only sit in class: you spend years on your own research and write a long piece of work that is supposed to add something new. When someone “has a PhD in history,” they finished that path. People also put PhD after a name, or use the title Doctor. Though actually, the letters come from Latin for “teacher of philosophy” — but today you can earn a PhD in chemistry, music, education, almost any field, not only philosophy. The “philosophy” in the name is an old use, meaning serious study. American writing often uses Ph.D. with dots; British writing usually uses PhD without them, and a few universities (Oxford is the famous one) say DPhil for the same kind of degree. In speech people say the letters one by one, P-H-D. Everyday talk: “She’s doing her PhD” means she is still on that course. It names a proud, formal achievement, but the letters themselves are ordinary in conversation.