This article proposes an instance-based theoretical framework to account for the influence of both contexts and definitions on learning new word meanings and reports 2 studies that examine hypotheses about learning from context. One is that variation in contexts is important for allowing core meaning features of a word to emerge. The second is that definitions are effective because they can interact with contexts to communicate core meanings. Both experiments tested the effects of context variation by presenting adult learners with context sentences that either varied or repeated with each training trial. Experiment 1 varied whether definitions were also provided, whereas Experiment 2 varied context variability without definitions and examined the role of reading comprehension skill and pre-training word familiarity. Results across several different measures were that exposure to variable contexts led to better learning of abstract meanings than did equivalent exposure to a single context. In addition, definitions were more effective at conveying this knowledge than context alone. The instance-based framework accounts for the dual effects of contexts and definitions, suggesting how word learning results from abstraction across varied word encounters, both definitions and context sentences. (Contains 4 tables and 1 footnote.)