Also sometimes referred to as crowdsourced science, the Oxford British and World English Dictionary defines citizen science as:
"The collection and analysis of data relating to the natural world by members of the general public, typically as part of a collaborative project with professional scientists."
The oldest citizen science project is the the Audubon Society's Christmas Bird Count, which was initiated in 1900. Today, citizen scientists participate in a wide variety of projects covering an array of scientific disciplines. Smart phone apps have made it much easier for people to contribute data.
To locate a citizen science project, see the Find a Citizen Science Project page. To learn more about how to researchers use data from such projects, as well as how reliable the data are, see the Citizen Science in Research page. This page also includes resources for researchers who want to begin a citizen science project of their own.