"People groups refer to groups of individuals, families and clans who share a common language and ethnic identity." (IMB) 
"A 'people group' is an ethnolinguistic group with a common self-identity that is shared by the various members." (IMB) 
"For evangelization purposes, a people group is the largest group within which the Gospel can spread as a church planting movement without encountering barriers of understanding or acceptance." -The Lausanne 1982 definition
"A people group is the largest group within which the gospel can spread along natural lines without encountering barriers of understanding or acceptance due to culture, language, geography, etc." -Ed Dayton
- Caleb Project - Infobytes Animations (Flash animation) - See module 3 for "What is a people group?"
Unreached people groups
An unreached people group is "a people group among which there is no indigenous community of believing Christians with adequate numbers and resources to evangelize their own people without outside (cross-cultural) assistance." -Frontier Missions 
An unreached people group is "a people group in which the number of evangelical Christians totals less than 2 percent of the population." -Scott Holste