the dissemination of information by an organisation so as to present an environmentally responsible public image
For example, a company that donates a small amount of money to an environmental charity, in return for a large amount of positive publicity, is guilty of greenwashing. Just think of the number of adverts that promote a company's 'green credentials' - a lot of them are 'greenwash'.
The term 'greenwashing' first appeared in the early 1990s and is a portmanteau (yes, another one!) of 'green' and 'whitewashing'.