Use coefficient in a sentence. You can multiply numbers by coefficients and use those numbers in equations to find the slope of lines and other data. When trying to decide how much of a product you will need you can use it as a coefficient and multiply it by the number of customers.
Allies refers to the countries that fought against German, Italy and Japan in World War II. An example of a country that was part of the Allies in World War II is Great Britain.
Objectively, one could say that American allies are those that have signed a mutual defence treaty with the United States. Those treaties vary quite a bit in their terms and obligations, but mostly obligate a signatory country to come to the aid of the other(s) if an attack occurs.