In functions, the coefficient is the number that multiplies the variables, for example, for 5y, the coefficient is the number "5"
The "leading coefficient" refers to the number that multiplies the greater term of the expression (i.e. the term with the largest exponent), and if the terms are ordered from greatest to least, it corresponds to the coefficient of the first term.
So for the expression
The first and the term with the largest exponent is:
Where "6" is the coefficient of the term and thus the "leading coefficient" of the expression.