What defines a "cousin"?

Earliest known ancestor for Walter is a John FAIRBAIRN, his son was John A Sr, married to Charity SMITHSON.
Clarissa is a descendant of David & Jane (WILLIAMSON) FAIRBAIRN, David being brother of William (marr. Jean WANLESS) FAIRBAIRN (see FAIRBAIRN Surname DNA project for link to Archibald (marr. Alison CROSSER) FAIRBAIRN).
Census data shows Walter as "cousin" to Clarissa's father David, but also David as "cousin" to Walter's father William George.
Yesterday I stumbled upon a reference to papers donated to the University of North Dakota by a Lawrence D FAIRBAIRN, born Montana and decided to investigate. He turned out to be the son of Angus U(rquhart?) FAIRBAIRN, and therefore a descendant of David and Jane (WILLIAMSON) FAIRBAIRNs. Further web searches led me to assorted Eatonville papers of Washington State that reported the daily goings on of the community, including that Mr and Mrs John A FAIRBAIRN of Hoquiam had been visiting, John being stated as cousin of Angus.
No surprise there, John and the above Walter being related, as were Angus and Clarissa.
But when I added a search of my previous correspondence about Hoquiam into the mix, I was reminded that a couple of years ago Joe and I had swapped info on the family of John in Hoquiam, and Joe had said that his grandfather Albert's 1938 obit stated that John A in Hoquiam was his cousin. Albert had emigrated from Liverpool to Hoquiam early 1900s, his line works back to a Robert who married an Elizabeth TAYLOR in Newcastle on Tyne 1811 but came from Sunderland himself.
Given Robert's son John is demonstrably in England until 1851 at least, he isn't a candidate for the John A family.
So "cousins" is obviously a loosely defined term by current usage, and presents an interesting puzzle to solve.
FAIRBAIRN web page updated to include a descendant chart for the Cousins of the Cousins.
Once again, any dna project takers?