” […] Imagine that you are sitting at a computer terminal. You type an *expression*, and the interpreter responds by displaying the result of its *evaluating* that expression. One kind of primitive expression you might type is a number. […]

Expressions representing numbers may be combined with an expression representing a primitive procedure (such as + or *) to form a compound expression that represents the application of the procedure to those numbers. For example:

(+ 137 349)486(- 1000 334)666(* 5 99)495 [...]

Expressions such as these, formed by delimiting a list of expressions within parentheses in order to denote procedure application, are called **combinations**. The leftmost element in the list is called the **operator**, and the other elements are called **operands**.

The **value of a combination** is obtained by **applying the procedure** specified by the operator to the **arguments** that are the **values of the operands**.”

— *Structure and Interpretation of Computer Programs (SICP)*, ‘1.1 The Elements of Programming‘