An omen (also called portent or presage) is a phenomenon that is believed to foretell the future, often signifying the advent of change. People in the ancient times believed that omens lie with a divine message from their gods.
These omens include natural phenomena, for example an eclipse, abnormal births of animals and humans and behavior of the sacrificial lamb on its way to the slaughter. They had specialists, the diviners, to interpret these omens. They would also use an artificial method, for example, a clay model of a sheep liver, to communicate with their gods in times of crisis. They would expect a binary answer, either yes or no answer, favorable or unfavorable. They did these to predict what would happen in the future and to take action to avoid disaster.
Though the word "omen" is usually devoid of reference to the change's nature, hence being possibly either "good" or "bad," the term is more often used in a foreboding sense, as with the word "ominous". The origin of the word is unknown, although it may be connected with the Latin word audire, meaning "to hear."
The oldest source for this practice in the Ancient Near East came from Mesopotamia. This practice attested at the first half of the 2nd millennium B.C., and it was vigorously pursued by the Assyrian kings, Esarhaddon and his son, Ashurbanipal in the 7th century B.C.
There were 3 methods to interpret omens, and they were hepatoscopy (i.e. liver divination), lecanomancy, and libanomancy. Hepatoscopy is to observe irregularities and abnormalities on the appearance of the entrails of a sacrificial sheep and they were used most in royal services.
Astrological omens were popular in Assyria, during the 7th century BC. Diviners gained much influence by interpreting the omens and advising the king how to avoid the terrible fate during the reign of Esarhaddon (681-669 BC). One of the things they would do in Assyria was to put a substitute king on the throne, and the true king would hide for a while. The substitute king was expected to take the evil consequences and when they believed the danger is over, they would execute the substitute king and the true king will be back on the throne.
The observations of omens were recorded into series. Some of them dated back to the first half of the 2nd millennium BC, and these were arranged as conditional statement later (if such and such is the case, then such and such is the result).