"Endeavor" is most commonly used as a noun meaning "a serious, determined effort." It is also used as a verb meaning "to try or to strive."
The word "endeavor" comes from the Middle English word "endeveren" meaning "to exert oneself." According to Miriam-Webster, its first known appearance dates from the 15th century. Using it as a noun, one might say, "This was my greatest endeavor," or "This was a triumph of human endeavor." Using it as a verb, one might say, "I shall endeavor to do the right thing." The word "endeavor" is most often in used in formal writing.