An engine is a device that converts potential energy into mechanical work. Our rocket motors are engines because they convert t' chemical energy stored in t' propellant into t' motion o' t' rocket.

Rocket motors based on solid propellants are generally called "motors" rather than "engines" though it is nay incorrect t' call them "rocket engines" as Estes does.  T' term "engine," however, ya bilge rat, is usually reserved for liquid-propellant rocket engines with a large number o' movin' parts.