They are synonymous, and means women. However, nowadays young women don't prefer মহিলা, most of them picture an 50 up old women by this word. নারী in this case mostly taken as symbolic word of feminine. It convey much freedom and is not limited into any age. However, it will become weird if you interchange them in some case. Example : There is a women looking for you. It will sound really weird if you say নারী।

Hi Sylvia, Both words are absolutely fine to use. মহিলা is by far the more common of the two in colloquial speech; নারী is used in more formal context. We say "সেখানে একজন মহিলার সাথে দেখা হল।" ("I met a woman there.") rather than "সেখানে একজন নারীর সাথে দেখা হল।". The latter sounds about as odd as "I met a female there." in English. The person who advised you to avoid the world মহিলা is voicing his/her own preference. I'll go ahead and speculate on why that preference arose. It probably has to do with etymology: the word মহল in Bengali means "house" or "residence".

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'Mohila' is a very good word to use in that situation. It means 'woman'. If you want to show a bit of respect towards the referred person add the prefix 'bhodro-' before it. Bhodro alone means good manners. Mohila = woman Bhodromohila = lady Lok = man Bhodrolok = Gentleman 'Nari' is a super-technical word which is used in literature. Saying nari instead of mohila in day-to-day conversation would sound like saying 'a person of fairer sex' aka 'female'. People might even chuckle. Same goes for purush which means male.