Definition from Nolo’s Plain-English Law Dictionary

1) Able to act in the circumstances, including the ability to perform a job or occupation, or to reason or make decisions. 2) In wills, trusts, and contracts, sufficiently mentally able to understand and execute a document. 3) In criminal law, sufficiently mentally able to stand trial or testify. 4) In evidence, relevant and legally admissible.

Definition provided by Nolo’s Plain-English Law Dictionary.