A notary public (a.k.a. notary or public notary; pl. notaries public) of the common law is a public officer constituted by law to serve the public in non-contentious matters usually concerned with general financial transactions, estates, deeds, powers-of-attorney, and foreign and international business. A notary's main functions are to validate the signature of a person (for purposes of signing a document); administer oaths and affirmations; take affidavits and statutory declarations, including from witnesses; authenticate the execution of certain classes of documents; take acknowledgments (e.g., of deeds and other conveyances); provide notice of foreign drafts; provide exemplifications and notarial copies; and, to perform certain other official acts depending on the jurisdiction. Such transactions are known as notarial acts, or more commonly, notarizations. The term notary public only refers to common-law notaries and should not be confused with civil-law notaries.

With the exceptions of Louisiana, Puerto Rico, Quebec (whose private law is based on civil law), and British Columbia (whose notarial tradition stems from scrivener notary practice), a notary public in the rest of the United States and most of Canada has powers that are far more limited than those of civil-law or other common-law notaries, both of whom are qualified lawyers admitted to the bar: such notaries may be referred to as notaries-at-law or lawyer notaries. Therefore, at common law, notarial service is distinctly different from the practice of law, and giving legal advice and preparing legal instruments is forbidden to lay notaries such as those appointed throughout most of the United States. Despite these distinctions, lawyers in the United States may apply to become notaries, and this class of notary is allowed to provide legal advice, such as determining the type of act required (affidavit, acknowledgment, etc.).