The word omen serves to define a form of divination and that frequently refers to the arrival of a change, therefore an omen represents a signal, which can be interpreted as the announcement of something that is about to happen. Omens can be considered good or bad, depending on how it is interpreted.
In ancient times, omens were phenomena that represented something really serious. Even in Greco-Roman times omens were classified as:
Those that arose from the trembling of the body or from palpitations. It is said that when a person has palpitations or trembles for no reason, it is a sign that something bad is going to happen.
The ringing in the ear is also considered an omen, as it is an indication that someone is talking about that person, in their absence.
The encounter with certain people, such as a dwarf, a cross-eyed or any other person with some physical defect, was considered an omen. The same thing happened if a person came across a black cat or a snake, since these animals were considered to be a bad omen.
There are some people who are dedicated to the art of divination, they usually interpret what is supposed to happen in the future; these people are known as clairvoyants or witches.
In ancient Roman and Greek peoples, always their inhabitants as well as their rulers, were aware of every sign they observed or sensation they felt, in order to determine what actions they should take. For them omens were sacred.
With the passage of time and thanks to the advancement of science and rationalism, the theme of omens was classified as superstitions that are only part of folklore and that do not affect in any way in the daily lives of people.
