Why the derivatives and gradient are used interchangeably?

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I am not good at mathematics and have been learning ML from Udacity.

In its tutorial video, the tutor mentioned

derivatives at time 4:24 https://youtu.be/9ILiZwbi9dA?t=264

and gradient across the whole video e.g. at time 3:03 https://youtu.be/9ILiZwbi9dA?t=184

So this makes me thinking derivatives and gradient are interchangeable.

Am I correct?

If not, what are the differences between Gradient and Derivative in laymen's language?(yes, I did searched the differences, but they are too mathematic. My hopeless not-math-capable brain cannot understand it but my curiosity keeps driving me to get it understood.... >_<)

and when to use gradient and when to use derivative?