Is there a good word for an idea that someone came up with on their own? I'm talking about an original idea. The reason I want such a word is for my notes so that I can annotate, with as few words as possible, whether I reached an idea on my own or whether I used someone else's. I prefer a formal term over informal.
You can consider brainchild. It is an informal word.
An idea or invention considered to be a particular person’s creation:
the statue is the brainchild of a local landscape artist
An original idea or plan attributed to a person or group.
You can consider inspiration as a more formal word.
a product of your creative thinking and work; "he had little respect for the inspirations of other artists"
Something, such as a sudden creative act or idea, that is inspired.
How about epiphany [Random House Kernerman Webster's College Dictionary]:
- a sudden, intuitive perception of or insight into reality or the essential meaning of something, often initiated by some simple, commonplace occurrence.
- a literary work or section of a work presenting such a moment of revelation and insight.