Sustainability a personal responsibility

Recently I was interviewing for a job position involving renewable energy when my interviewer asked me a question I've never really been asked before: "What does sustainability mean to you?" To be honest, I tanked this question. I thought about all the times any professor, lecturer or die-hard environmentalist spoke of sustainability, without providing a clear-cut definition for it. I spattered about leaving carbon footprints and how being wasteful was contaminating the environment, but knew I was skirting around the core of the birthing of sustainability.