The traditionally correct phrase (adverb) is to use ‘by accident’. It means by mistake or something that’s done without the intention of doing it. For example, “she spilled the milk by accident.” We can also use the word ‘accidentally’ as a replacement for by accident – just two ways of referring the same thing. Some other synonyms of the word include unintentionally, by chance.
On Accident vs. By Accident
A less commonly used variant of by accident, on accident is almost exclusively used in the United States. Even there, no one uses ‘on accident’ in writing, its only a spoken English term. In fact, it is considered incorrect to be used in written form.
Well, most younger people tend to use on accident on similar lines of ‘on purpose’ although the meaning is completely opposite. The word ‘on’ is the confusion creator.
So, we can say – “The glass slipped from my hand on accident”, however it just doesn’t seem or sound right.
However, when there are other words following or in between on accident (like on _____ accident), the context of the sentence changes and it would be right to use the same. For example:
As we see, on accident cannot be used as an adverb phase, both on and accident are used as independent words in sentences. Whereas, ‘by accident’ is the only correct phrase to indicate unintentionally or accidentally.