The present review explores the meaning of the term 'incidental' in light of how incidental learning is framed, conceptualized, and operationalized in second language (L2) vocabulary research. Three interpretations of incidental vocabulary learning that seem to appear recurrently in the literature are presented and discussed along with examples of empirical studies. These interpretations are labeled as: (1) learner-oriented, (2) method-oriented, and (3) pedagogy-oriented definitions. Significant constructs and theories in the field of second language acquisition (SLA) that motivate these interpretations are also considered. The paper concludes with a brief summary of current views on incidental learning and suggestions for future research.