Second-hand smoke is the smoke exhaled by you, plus the smoke created by the lit end of a cigarette. People who breathe in second-hand smoke are at risk of the same diseases as smokers.
Source: Secondhand smoke @ NHS Smokefree
Image by Challiyil Eswaramangalath Vipin from Chalakudy, India (cigarette.. like a cigarette in the rain) [CC-BY-SA-2.0], via Wikimedia Commons
