Collision and Collusion are easily confused words.
The spell-check application of most word processing software programs would not catch a slip-up of these two words. Spell-check is looking for words that aren’t in its dictionary, and words that resemble words in its dictionary but are possibly spelled wrong. Spell-check isn’t perfect. It doesn’t know and can’t guess what word you wanted or what word you meant, it can only judge the words on the page. If you used words that are all spelled correctly, it gives you the “okay” though typos may still be present.
Autocorrect, a similar app, also tries to anticipate what word you want based on the first few letters. But words sharing the first few letters doesn’t mean the words are synonymous, or have similar meanings.
Collision is a noun. It means when two objects strike, or run into one another.Collision is the noun form of the verb “collide.” Collision is frequently used to describe car, bicycle, and other vehicular traffic accidents.
Collusion is a noun. It means who two or more persons agree (in secret) to commit a criminal act, like fraud. Collusion is the noun form of the verb collude.
The following story uses both words correctly:
Colleen couldn’t believe her bad luck, after being involved in a serious collision, her car insurance company accused her of being in collusion with the other driver. They suspected both drivers wanted to get new cars and felt the quickest way to do it was to total the older models they owned. “Who would risk bodily harm, inconvenience, and lost income just to get a new car?” she wondered, “that’s a lot of long term pain for so little benefit.”