Access to definitions facilitates the learning of word meanings when novel words are encountered in reading. However, the memorial costs and benefits of inferring word meanings from context, compared to seeing definitions of unfamiliar words before reading, are not yet well understood. We conducted two experiments with adult L1 (English) and L2 (Chinese) readers to investigate whether the development of declarative and nondeclarative word knowledge benefits more when definitions are supplied before reading (errorless treatment) or after reading (trial-and-error treatment). Study participants encountered 90 target vocabulary items three times in short informative texts under errorless or trial-and-error conditions and entered their meaning inferences immediately after reading each text. Posttreatment, we evaluated participants' declarative knowledge of the target items using a meaning generation (recall) task and nondeclarative knowledge using a self-paced reading task. The trial-and-error treatment followed by definitions resulted in a superior declarative and nondeclarative knowledge, compared to the errorless treatment, for L1 and L2 readers. Inference errors affected the development of declarative but not nondeclarative knowledge, and the trajectory of the development of nondeclarative knowledge was different for L1 and L2 readers. We interpret these findings in terms of the declarative and nondeclarative memory processes underpinning contextual word learning.