"A constraint that establishes a parent-child relationship between two tables via one or more common columns. The foreign key in the child table refers to a primary or unique key in the parent table." (Bob Bryla, "Oracle Database Foundations", 2004)
"A database object that enforces data reference integrity between (or within) tables." (Sara Morganand & Tobias Thernstrom , "MCITP Self-Paced Training Kit : Designing and Optimizing Data Access by Using Microsoft SQL Server 2005 - Exam 70-442", 2007)
"A database object (data integrity mechanism) that maintains referential integrity by establishing and enforcing a link between the data in two tables." (Marilyn Miller-White et al, "MCITP Administrator: Microsoft® SQL Server™ 2005 Optimization and Maintenance 70-444", 2007)
"A foreign key constraint is a logical coupling of two SQL tables through the values of specified columns." (Michael Coles, "Pro T-SQL 2008 Programmer's Guide", 2008)
"A logical coupling of two SQL tables through the values of specified columns." (Miguel Cebollero et al, "Pro T-SQL Programmer’s Guide" 4th Ed, 2015)
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