One definition is "duties recommended, but not essential; fulfilment of which is rewarded, though they may be neglected without punishment".
Synonyms of mustahabb include masnun and mandub.
The opposite of mustahabb is makruh (discouraged).
There are thousands of mustahabb acts, including:
- As-Salamu Alaykum (a traditional Islamic greeting) (though responding to the greeting is an obligation)
- Sadaqah (charity outside of zakat)
- Umrah (except in the Shafi'i madhhab, wherein it is fard)
Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Mustahabb.