The noun form of the word 'Spontaneous' is -

• The noun form of the word 'Spontaneous' is - Spontaneity. 

• Spontaneity: 
- noun. 
English meaning:
- the quality of being natural rather than planned in advance. 
বাংলা অর্থ: স্বতঃস্ফূর্ততা। 

• Spontaneous:
- (adjective).
বাংলা অর্থ: স্বতঃস্ফূর্ত। 

Source: Accessible dictionary by Bangla Academy, Cambridge Dictionary, merriam-webster Dictionary.