A biotope is an area in nature that defines a particular habitat and is usually bound by species types, or by geographical features.
A biotope may be only a particular portion of an ecosystem; Lake Malawi for instance, contains many hundreds of cichlid species although significant portions are only found in the shallow rocky areas. These rocky areas could be considered a 'biotope' even though they are part of a larger system.
In fishkeeping 'biotope' is used to describe an aquarium designed to represent a portion of nature e.g. a 'Lake Malawi rocky biotope', an 'Indonesian swamp biotope', or an 'Amazon acid pool biotope'.
In a true biotope aquarium, the plants and fish are all chosen on the basis of the area they originate in, and the décor or aquascape, is designed to represent that area, or biotope.
Used a little more loosely, but still in a functional manner, a biotope aquarium may use fish and plants which are not found together in nature to produce a 'riverbank biotope', or a 'fast-flowing stream biotope'.