互 is a bound morpheme (b.f.) meaning 'mutual' or 'each other'. It cannot stand alone as a word — you must combine it with another morpheme (usually a single character) to form a compound like 互相 ('each other, mutually'), 互助 ('mutual aid'), or 互利 ('mutually beneficial'). Think of it as a prefix that turns the base word into a reciprocal relationship. In writing, it often appears in formal or semi-formal contexts.