专业(zhuān yè) - university subject, speciality; professional, specialist in Mandarin

专业 (zhuān yè) wears two hats. As a noun it is your university subject, what an American calls a major and a Briton calls a degree course, and 你学什么专业 is one of the first questions a Chinese student will ask you. As an adjective it means professional in the sense of expert: 专业的老师, 很专业.

That adjective sense is about competence, not about being paid. A brilliant amateur cook can be described as 很专业, and a paid worker who is sloppy is 不专业. If you specifically mean paid rather than amateur, the word is 职业, as in 职业运动员 (professional athlete).

Do not confuse it with its neighbours: 事业 is a career or life's work, 职业 is a profession, 专家 is an expert. And 专业 modifies with 的 like an ordinary adjective, so the phrase is 专业的意见 (a professional opinion), not 专业意见 in careful speech, though the shorter form is common in headlines.