Most of us think of cousins in terms of our aunts' and uncles' kids, or 2nd and 3rd cousins. Trying to figure out where cousins fit can be a challenge - 1st or 2nd cousin once removed, 3rd or 4th cousin, etc.
For example if two of you hare the same great-great-grandparents you are 3rd cousins. Your 1st cousin's kids are 1st cousins once removed.
In genealogical research, "cousin" can refer to any person related by blood or marriage. Same definition we use today, but without the detail. We might refer to "my uncle's sister-in-law" today. 300 years ago, our ancestor might have referred her as simply "cousin."
That leads us to relationships in research, especially wills. Trying to sort out who is related to whom and how can be a challenge with the term cousin, niece or nephew. All three can take on a broader scope than we think of in today's terms. They can be used interchangeably as well.
Case in point: Ancestor Bridget Very Giles is referred to as "my cousin Bridget Giles" in the will of Henry Scudder, Sr.  Some researchers had determined Bridget was Henry's daughter, but he names a daughter Bridget Graile in the will.
The question arises from the will, how was Bridget related to Henry Scudder, Sr.? Speculation ranges from her being the daughter of Henry's brother, Thomas, to being the child of one of Henry's daughters. The daughters are named in the will: Jane Russell, Martha Jacob, Bridget Graile and Elizabeth Tombes, dec'd. Bridget could have been the daughter of one of Henry's other siblings or cousins as well.
For now, I have left Bridget, erroneously, as Bridget Scudder in my files. Was her maiden name Scudder, Russell, Jacob, Graile, Tombes or that of another married Scudder female? We do know she is of Scudder blood.