A rural community which has the ability to smoothly absorb tens or hundreds of times its baseline population for special events or disaster relief purposes.
The core idea is to over-provision infrastructure like drinking water supply, sewage systems and so on to be able to handle much large populations than live year-round on the land, while using some kind of rapidly scalable shelter system to accommodate the peak populations, like tents or the hexayurt. A convertible community may offer disaster response places on a subscription basis in high risk areas.
http://www.appropedia.org/Convertible_community - origination of the concept
SERAA - uses convertible communities to absorb displaced populations from natural disasters, also ReDs support