Nipple dip is when the nipples have a dip or slit in them.
(You can also think of the shape of a red blood cell for example)

Examples of nipple dip:
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Examples of no nipple dip:
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Examples of strong nipple dip:
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Examples of weak nipple dip:
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Examples of nipple dip through clothing:
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Nipple dip is not to be mistaken with inverted nipples, though in some cases both can be in the same image.
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Nipple dip is also visible from the side, or even back view.
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Nipple dip isn't limited to only females:
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