In programmer terminology you have created a variable. It is called a variable because you can vary its contents. Whenever you need the value again you simply recall the variable by name, and its current contents are returned to you. AutoLISP is not case-sensitive, so RAD, Rad, and rad are the same. You can have as many memories as you need, simply by giving them suitable names.
Variable names can use letters, numbers, and most punctuation marks, with the following limitations.
They cannot consist of just numbers. The variable name 34 is not legal, but 3A is. Okay, invoke the Circle command again and pick a center. To recall the value assigned to a variable at the Command prompt or within an AutoCAD command, put an exclamation mark in front of its name. Along with arithmetic operations and internal functions to calculate logs, trigs, exponents, etc., AutoLISP makes logical comparisons such as less than, equal to, greater than, and other equality and conditional evaluations.