"coefficients" are usually constants by which variables or functions are multiplied.
In the binomial 2x + 3, the coefficients are 2 and 3 for x an 1 respectively.
In the trinomial 4x^3 - 2x, the coefficients are 4, -2 and 0, since there is no constant term (meaning that the constant term is actually zero (0) 0.
In the trig expression 4 sin x - 2 sin y, 4 and 2 are the coefficients of sin x and sin y respectively.
