[brēy·wan ¦fəŋk·shən] (solid-state physics) A function of x with index (or parameter) n that appears in the quantum-mechanical theories of paramagnetism and ferromagnetism and is expressed as [(2 n + 1)/2 n ] coth [(2 n + 1) x /2 n ] - (1/2 n) coth (x /2 n).