Of the Dictionary.com definitions "nonperformance of something required or expected" resonates most with me.
Failure is not trying. Not changing. Not progressing.
I write about failure a lot because as creatives, we put so much of ourselves into our work that it's hard to separate yourself what you've done isn't well received.
Remember: Just because this particular idea/execution may not be right doesn't mean there's anything wrong with you.
Fall down 7 times, get up 8.
Failure is staying down.
Related Reading: Fear of Failing, How To Fail, Fail Big, Make Mistakes, Don't "I don't know" me,
What if, Make it Up, Dealing With Death