The amount of carbon dioxide (CO2) emitted during an industrial or domestic process; a measure of how that process contributes toward global warming.
A spam e-mail that I received in April 2007 encouraged me to diet at Easter by 'offsetting my chocolate footprint'. That's an indication of how quickly the recent interest in the environment is making its presence felt in the language. Not so long ago a 'footprint' was just that - the print made by a foot. News stories now use 'footprint' as shorthand for 'carbon footprint'.
It was space exploration that gave the word a new lease of life, not via the moon landings and the celebrated 'one small step for man', but as a term indicating the oval area that a spacecraft aimed to land in. That was defined in 1965 in the Science Year newsletter:
"Footprint, the proposed landing area for a spacecraft."
Following that, the field of computing took up the term and before long there were footprints on our desks. The December 1982 edition of Computerworld magazine referred to the 'desktop footprint':
"One area is the introduction of terminals with smaller footprints that are designed for the executive's desk."
This introduced the idea of the effect of the PC on our desktop and the notion that 'small footprint good; large footprint bad'.
The term 'carbon footprint', otherwise expressed as the 'global footprint' or 'ecological footprint', is now widely used, although possibly less so in the USA than in other parts of the English-speaking world. The term was introduced to a UK audience in the early years of the new millennium; for example, this piece from the Welsh newspaper The Western Mail, July 2005:
"First Minister Rhodri Morgan tried out the Centre for Alternative Technology (CAT)'s Carbon Gym yesterday and was pleased to see the size of his 'carbon footprint' was below average, meaning he has a fairly green lifestyle."
Footprints are creeping into some very odd places. My guess is that 'chocolate footprints' won't be the end of it.