Incorrect spelling, explanation: there is no double s in decision, due to the word’s origin (Latin decisionem from verb decidere). Therefore decission is an incorrect spelling. The only correct form is decision.

Correct spelling, explanation: decision is the correct spelling because the word comes from Latin, decisionem, in which the word is also spelled with one s. Therefore decission is an incorrect form and decision is the correct one.

Definition of decision:
noun, an act of deciding or making a conclusion after thinking about a problem
I can’t make a decision about this issue without consulting my boss.
Come on! Choosing a prom dress is not such a hard decision, is it?

Incorrect spelling

Incorrect spelling

Incorrect spelling