"I am an Instructional Designer." Blank stares and the occasional, "Huh?," are the usual responses after the conversation circle has established that the guy to the right of you is an Accountant, the woman to the right is a Physical Therapist and the two across from you are business owners. Everyone seems to grasp those common labels pretty quickly. Instructional Design though? You can probably see them mentally scratching their heads and trying to come up with any past association that provides some recognition.
And, to be honest, it is not much easier to describe it. I usually end up flipping the words around to say, "I design instruction or learning." If blank looks prevail, I pare it down to "Training, I design training," usually followed with a (mostly inaudible) sigh from me. But really, it is so much more. In a nutshell, we try to find the best ways to help people learn what they need to learn, whether that is to work safely at a construction site, be a better leader, design a website, or even create the perfect sandwich. As hard as it is for some to believe, more training is usually not the answer to the performance question.