1. The circumstances or conditions that surround one; surroundings.
2. The totality of circumstances surrounding an organism or group of organisms, especially:
a. The combination of external physical conditions that affect and influence the growth, development, and survival of organisms:
b. The complex of social and conditions affecting the nature of an individual or community
My own definition:
The surroundings around us, the changes that take place around us whether man-made or natural.
The conditions that affect an organism's lifestyle.