A constant is any number except letters, including positive and negative integers, positive and negative decimals, fractions and 0. A mathematical constant refers to a constant with a constant value, and vice versa. Unlike most physical constants, the definition of mathematical constants has nothing to do with all physical measurements.
The degree of a single item is the exponential sum of letters, and the constant item has no letters, so the degree is 0. As for the number of constant terms, it can also be understood as follows: match a constant with a letter factor with an index of 0 that is not equal to 0 (nonzero zeroth power is equal to 1). Obviously, the number of constant terms is 0.