"Y is functionally dependent on X if the value of Y is determined by X. In other words if Y = X +1, the value of X will determine the resultant value of Y. Thus, Y is dependent on X as a function of the value of X. Functional dependence is the opposite of determinance." (Gavin Powell, "Beginning Database Design", 2006)
"A constraint between two attributes or two sets of attributes in a relation." (S. Sumathi & S. Esakkirajan, "Fundamentals of Relational Database Management Systems", 2007)
"A relationship between two attributes (or two combination of attributes) in a relation such that for every unique value of the second attribute, the table contains only one value of the first attribute. The first attribute, however, may be associated with multiple values of the second attribute." (Jan L Harrington, "Relational Database Design and Implementation" 3rd Ed., 2009)
"The property of one or more attributes (data items) that uniquely determines the value of one or more other attributes (data items). Given a table R, a set of attributes B is functionally dependent on another set of attributes A if, at each instant of time, each A value is associated with only one B value." (Toby J Teorey, ", Database Modeling and Design" 4th Ed., 2010)
"Within a relation R, an attribute B is functionally dependent on an attribute A if and only if a given value of the attribute A determines exactly one value of the attribute B. The relationship 'B is dependent on A' is equivalent to 'A determines B' and is written as AB." (Carlos Coronel et al, "Database Systems: Design, Implementation, and Management" 9th Ed., 2011)
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