I'm sorry about the title; I lost a bet with the '90s.
Below are five categories. Each category is united by one word in the category; furthermore, each category rule is formed in the same way as the other category.
Find the word in each category that holds the category together.
pilgrim, car, antirealist, brotherhood, thornbush
predator, abluent, dastard, flag, oilcloth
plush, math, terminus, basinet, splotch
music, nanotech, barbell, steppe, country
piranha, woman, peruse, formality, map