A constant in logic is a symbol which refers to a specific object. It is essentially a proper noun in a logical language.
By convention, lowercase Roman numerals are used to represent constants. If one wants to refer to a specific person, for example, Wanda, one may use the lowercase letter, w.
Constants are usually used in association with predicate symbols, which add semantic meaning to logical expressions. For instance, we may define the predicate symbol Ex as “x eats”, where x is a variable. Then, we may say Ew to express “Wanda eats”.