In general a Ninja Edit is a change made to a published post or comment that preempts the first response made. The phrase is composed of "Ninja", a much-revered warrior caste famed for their speed and agility, and "Edit", the process of changing a thing.
On Reddit specifically it is any edit to a post/comment that happens within the first 3 minutes after submission it will not be marked as edited. It is kind of like a grace period revise a post/comment.
References
"This is what a well performed ninja edit looks like"
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| A friendly non-sneaky ninja editing her comment from the shadows |
Flair: Reddit
Collection: Reddit
