Sustainability is the ability to use resources in an efficient and responsible way [efficient: not wasteful, responsible: taking care of something] to meet the needs of the present generation without compromising the needs of future generations. For example, using renewable [able to be replenished] energy sources such as solar or wind power instead of fossil fuels.
Very Simple Definition:
Sustainability means using resources in a way that does not harm the environment and can be used for a long time. For example, recycling paper and plastic instead of throwing them away [recycling: using materials again].