Understanding the "between" lunar phase names is easy if you break out and define 4 words: crescent, gibbous, waxing, and waning. The word crescent refers to the phases where the moon is less than half illuminated. The word gibbous refers to phases where the moon is more than half illuminated. Waxing essentially means "growing" or expanding in illumination, and waning means "shrinking" or decreasing in illumination.
The Waning Gibbous moon is the phase when the moon is becoming less visible: After the full moon (maximum illumination), the light continually decreases.