Traditionally an oath is either a statement of fact or a promise taken by a sacrality as a sign of verity. A common legal substitute for those who conscientiously object to making sacred oaths is to give an affirmation instead. Nowadays, even when there is no notion of sanctity involved, certain promises said out loud in ceremonial or juridical purpose are referred to as oaths. "To swear" is a verb used to describe the taking of an oath, to making a solemn vow.
Depiction of the legendary Swiss Rütli Oath taken by the representatives of the three founding cantons, declaring unity and freedom in eternal alliance
"Tennis Court Oath" by Jacques-Louis David.
The oath of the provisional triumviral regents of the Empire of Brazil in the Imperial Chapel in 1831, during a period known as the Período regencial.
Isabel, Princess Imperial takes oath as regent of the Empire of Brazil before the Imperial Senate, c. 1870.
A promise is a commitment by someone to do or not do something. As a noun promise means a declaration assuring that one will or will not do something. As a verb it means to commit oneself by a promise to do or give. It can also mean a capacity for good, similar to a value that is to be realized in the near future.
American immigrants seen swearing the Oath of Allegiance in 2010
A painting depicting Cola di Rienzo as vowing to obtain justice
Diplomats from Switzerland making promises at a meeting in the Philippines
Linking little fingers is one of several common gestures indicating a promise has been made.