From Appropedia, the sustainability wiki:

Appropriate technology (AT) is technology that is designed to be "appropriate" to the context it is intended for - including environmentally, culturally and economically.

The main characteristics of most ATs are that they should be sustainable, small and appropriate. To be effective in practice, it should be easy to maintain. To be effective for many people and have a wide impact, it must be affordable.

It can also be described as the simplest level of technology that can effectively achieve the intended purpose in a given location. In developed countries, the term appropriate technology takes a different meaning, often referring to engineering that takes special consideration of its social and environmental ramifications.

Some prefer to capitalize the term as Appropriate Technology, to distinguish it from other situations where the words "appropriate technology" might be chosen. Synonyms are intermediate technology, simple technologies and technologies for the poor.