"Notice that Lisp is an object-oriented language in the sense that it evaluates objects, not expressions." Common Lisp (Shapiro)
On searching for definitions of both elements of the dicotomy:
OBJECT:
In computer science, an object is any entity that can be manipulated by the commands of a programming language, such as a value, variable, function, or data structure. (With the later introduction of object-oriented programming the same word, "object", refers to a particular instance of a class)(Wikipedia)
EXPRESSIONS:
An expression in a programming language is a combination of explicit values, constants, variables, operators, and functions that are interpreted according to the particular rules of precedence and of association for a particular programming language, which computes and then produces (returns, in a stateful environment) another value. This process, like for mathematical expressions, is called evaluation. The value can be of various types, such as numerical, string, and logical. (Wikipedia)
Both share some elements, a value or a variable can be both an object or an expression. So, what's the difference?