1.when one's reactions are unusually fast, like that of a ninja's.
2. when one seems to appear out of nowhere, often "appearing" in the middle of a conversation between two other individuals.
1.*drops phone and catches it in mid air*
Bob: "DUUDEE! nice catch!"
Jimmy: "i know,that was a total ninja moment."
2. Ellie: "...that test was really difficult! I left loads of questions out!"
Sophie: "i left out about five questions."
Emily: "i left out three"
Ellie: "woah! emily, where did you come from!"
Sophie: "ninja moment."