osnov

1. (in official documents) legal ground; basis
   Imam privremeni boravak po osnovu zaposlenja. — I have temporary residence on the basis of employment.
   Koji je osnov vašeg boravka u Srbiji? — What is the basis of your stay in Serbia?
2. grounds; basis
   Nema nikakvog osnova za takvu optužbu. — There are no grounds for such an accusation.
   Njegov strah nije bez osnova. — His fear is not unfounded.
3. (often in the plural) foundation; fundamentals
   Poverenje je osnov svakog dobrog odnosa. — Trust is the foundation of any good relationship.
   Na prvoj godini se predaju osnovi ekonomije. — In the first year they teach the fundamentals of economics.

Forms and payment slips often use "po osnovu" + gen., meaning "on the basis of, in respect of". In the general sense 'foundation' the feminine form osnova is more common; osnov is usual in legal texts and in the plural (osnovi ekonomije).