Incorrect spelling, explanation: this is a mistaken form. Senior is a word created by combining Latin senex (old man) and the comparative suffix -or. Therefore senoir is incorrect. Additionally, this spelling would suggest different pronunciation (something along with [se-nuh-wa:]).

Correct spelling, explanation: this word comes from a Latin word meaning old man: senex with adjective comparatives suffix -or added. This fact points to the spelling senior. Moreover, senoir would be pronounced differently: [se-nuh-wa:], while senior is pronounced [seen-yer].

Definition of senior:
noun, someone who is older than another; the oldest class in the school
There are cheaper tickets for seniors in this cinema.
He is a senior and has got his prom this year.
adjective, older or elder than someone in the family, usually with the same name as the younger person (for example when the son gets the father’s name); someone in the company in a higher position
This is Jack, and this is his father, Jack senior.
Mary got a promotion and she is now a senior developer.

Incorrect spelling

Incorrect spelling

Incorrect spelling