Here are two renowned and authoritative psychology dictionaries accessible online as eBooks (library login required off campus). After you open these eBook dictionaries, use the "search within" feature to look up terms.
A Dictionary of Psychology by 
APA Dictionary of Psychology by 
Biographical Dictionary of Psychology by 
Companion Encyclopedia of Psychology by 
Diagnostic and Statistical Manual of Mental Disorders (DSM-5) by 
Encyclopedia of Adult Development by 
Encyclopedia of Human Emotions by 
Encyclopedia of Human Intelligence by 
Encyclopedia of Mental Health by 
Encyclopedia of Psychology by 
Encyclopedia of Schizophrenia and Psychotic Disorders by 
Graduate Study in Psychology by 
Handbook of Child Psychology (4 vols.) by 
Handbook of Clinical Child Psychology by 
Measures for Clinical Practice: A Sourcebook by 
Tests: A Comprehensive Reference for Assessments in Psychology, Education, and Business by 
The Encyclopedia of Human Development and Education by 
The Gale Encyclopedia of Psychology (eBook) by 
Thesaurus of Psychological Index Terms by 
