Free-Body Diagram
Definition
A simple sketch showing an object alone with arrows for every force acting on it — the essential first step in solving force problems.
Technical definition
A diagram in which the system of interest is isolated and all external forces are drawn as vectors originating from a single point (or the object's center). Internal forces are omitted. It is the graphical encoding of the Newton's second law setup.
Example
For a book on a table, the free-body diagram shows two arrows: gravity pulling down and the normal force pushing up — equal in size, so the book stays put.
Only draw forces acting ON the object — never forces the object exerts on other things. A common error is including both action and reaction forces on the same diagram, which double-counts and breaks Newton's second law.