Boyd’s Bible Dictionary:

Appeals to God to attest the truth of an assertion in early use
(Gen. 21:23; 26:323Now therefore swear unto me here by God that thou wilt not deal falsely with me, nor with my son, nor with my son's son: but according to the kindness that I have done unto thee, thou shalt do unto me, and to the land wherein thou hast sojourned. (Genesis 21:23)
3Sojourn in this land, and I will be with thee, and will bless thee; for unto thee, and unto thy seed, I will give all these countries, and I will perform the oath which I sware unto Abraham thy father; (Genesis 26:3)
; Heb. 6:1616For men verily swear by the greater: and an oath for confirmation is to them an end of all strife. (Hebrews 6:16)). Regulated in Ex. 20:77Thou shalt not take the name of the Lord thy God in vain; for the Lord will not hold him guiltless that taketh his name in vain. (Exodus 20:7); Lev. 19:1212And ye shall not swear by my name falsely, neither shalt thou profane the name of thy God: I am the Lord. (Leviticus 19:12). Forms: lifting hands (Gen. 14:2222And Abram said to the king of Sodom, I have lift up mine hand unto the Lord, the most high God, the possessor of heaven and earth, (Genesis 14:22)); placing hand under thigh (Gen. 24:22And Abraham said unto his eldest servant of his house, that ruled over all that he had, Put, I pray thee, thy hand under my thigh: (Genesis 24:2)); before the altar (1 Kings 8:3131If any man trespass against his neighbor, and an oath be laid upon him to cause him to swear, and the oath come before thine altar in this house: (1 Kings 8:31)); laying hand on the law.

Strong’s Dictionary of Greek Words:

from a compound of 3727 and a derivative of 3660; asseveration on oath
KJV Usage: