a solemn promise, usually invoking a divine witness, regarding your future acts or behavior
"they took an oath of allegiance"
A solemn pledge or promise that invokes a deity, a ruler, or another entity (not necessarily present) to attest the truth of a statement or sincerity of one's desire to fulfill a contract or promise.
"The judge administered an oath requiring him to tell the truth before testifying in court."
In plain English: An oath is a serious promise to tell the truth or do something important, often made while holding your hand up and speaking loudly so everyone can hear you.
"He took an oath before serving on the jury to tell the truth."
Usage: Use "oath" specifically when invoking a higher power or authority to guarantee truthfulness, whereas "vow" is preferred for personal commitments without such external witnesses. Avoid using it casually; this term carries a formal weight that distinguishes it from everyday promises.
To pledge.
"The judge asked each witness to take an oath before testifying."
The word "oath" comes from the Middle English ooth, which traces back to the Old English āþ meaning a solemn promise. It is related to many other Germanic languages that use similar words for an oath, such as the German Eid.