1. [adj]  showing the way by conducting or leading; imposing direction on; "felt his mother's directing arm around him"; "the directional role of science on industrial progress"
2. [adj]  relating to or indicating directions in space; "a directional microphone"
3. [adj]  relating to direction toward a (nonspatial) goal; "he tried to explain the directional trends of modern science"

DIRECTIONAL is a 11 letter word that starts with D.