What is Mustahab?
Lexically, it means “something that is liked”. As a term, it means something that our Prophet occasionally did; for instance, the prayer of duha (late morning).
Our Prophet liked the things that are mustahab and did them from time to time. Salaf Salihin (the early generations of Muslims) did them gladly and encouraged other believers to do them.
Mustahab is called sunnah ghayr muakkada by some scholars; it is also called mandub, nafila, tatawwu’, adab.
Especially in fiqh book, the word adab is often used instead of mustahab.