A biological community that is in a stable state. The local ecology is stable, populations are holding steady (or have fallen into natural and sustainable cycles), and no great threat is looming. This is the 'final' seral stage, but only in the sense that all seres build towards this. Something will come along to disrupt things eventually, and the cycle will start over.
Climax communities may be prairies, forests, rainforests, tundra, deep ocean, desert, marshes, and most other classic types of ecosystems. But any of these systems may fall into disarray, because of fire, human intervention, an ice age, the end of an ice age, a volcano, an invasive species, or some other destructive event. Once this happens pioneer species must move back into the area, beginning the process of ecological succession, until eventually a new, potentially quite different, climax community is reached.
If a climax community is very large and well defined, it may be referred to as a biome.