Refers to a person or entity that hires and pays workers. Neutral, standard term.
新しい雇用主はとても親切だ。
My new employer is very kind.
雇用主は従業員に給料を支払う義務がある。
Employers have an obligation to pay wages to their employees.
雇用主 and 雇い主 both mean 'employer', but 雇用主 sounds more formal and is favored in legal or official documents.
Compound of 雇用 (koyō, 'employment') and 主 (nushi, 'master' or 'owner').