An ecological niche is an organism's specific in an ecosystem.

An ecological niche refers to an organism's specific behavior in its specific ecosystem. 

There are many things that fall under this description. For example, by specifying an ecological niche, we could be talking about the behavior an organism is exhibiting when there is plenty of resources or when the resources in a certain ecosystem are scarce. This has to be taken in account together with the question whether he has to compete with other organisms in his ecosystem for those resources. 

Hope this helps :)