From treatobacco.net, the best definition for Harm Reduction would be:
Harm reduction approaches covered by recent guidance offered by the National Institute for Health and Care Excellence (2013) include: stopping smoking (using licensed nicotine containing products for as long as needed), cutting down prior to stopping smoking (with or without out licensed nicotine containing products), smoking reduction (with or without out licensed nicotine containing products), and temporary abstinence from smoking (with or without out licensed nicotine containing products). The guidelines note that at this time “it is not clear whether there are any long-term health benefits from adopting a smoking reduction approach (apart from the fact that someone who reduces may ultimately stop smoking) (page 31) (NICE, 2013). However, there were shorter term benefits highlighted including improved physical and mental wellbeing and reduced exposure to smoke for non-smokers. In addition, for smokers who are ill, stopping or reducing the amount they smoke can mean they have more time to participate in therapeutic activities.