This resource was developed by Child Trends researchers as a one-stop source of clear and common definitions for terms that may be encountered by practitioners who serve youth and young adults. Some of these terms will be familiar; others may be new. We expect that a better understanding of these terms related to youth and young adult development can be useful for practitioners who work day to day with young people, for grant writers describing contexts and interventions as they seek funds, or for program managers seeking to update programs or train staff.