Omens may be considered "good" or "bad", but the term is more often used in a foreboding sense, as with the word "ominous".

In ancient Rome

Ancient Roman religion employed two distinct types of professional omen readers. Augurs interpreted the flights of birds, while haruspices employed animal sacrifice to obtain the entrails necessary for divination.

Astrology

See also: Eclipse cycle, Metonic cycle, Saros cycle, Comets

In the field of astrology, solar and lunar eclipses (along with the appearance of comets and to some extent the Full Moon) have often been considered omens of notable births, deaths, or other significant events throughout history in many societies. For example the Magi in the Gospel of Matthew predict the birth of Jesus after seeing the Star of Bethlehem.

Good or bad

Omens may be considered either good or bad depending on their interpretation. The same sign may be interpreted differently by different people or different cultures.

For example, a superstition in the United States and other countries across Europe indicates that a black cat is an omen of bad luck.

Comets also have been considered to be both good and bad omens. The best-known example is probably Halley's Comet, which was a "bad omen" for King Harold II of England but a "good omen" for William the Conqueror.

Omens in Indian Astrology

POV: date=April 2008 Omenology is called Nimmita or shukuna shastra in Vedic Astrology

Omen seen or heard or even visualized at the initiation of an activity, are said to foretell the outcome of the activity. Omens & portents (Shakun & Utpaats) is a useful branch of India astrology, which includes interpretation of dreams, status of living & non-living items in the environment, sounds produced by human & animals, portents, mode of pacification of adverse omens & portents. It acts as a guide in horary astrology, to clinch the issue when there is a stalemate. Coming events cast their shadows before & it is the ingenuity & skill of the interpreters to decode omens correctly for their profitable usage in their daily life.

Treatises on omen (Shakun) have commended that omen has the final say in Election. Omens seen at the start of an action does foretell its success. In case adverse omen is seen or heard or even visualized; the activity should not be initiated.

Omen is a wonderful knowledge which acts like a medicine. Vasant Raj in his treatise titled ‘Vasantraj Shakunan' - an authoritative book on the subject - has opined that when ephemeral elements (Tithi, Nakshatra, election ascendant etc.) are fully auspicious & efficacious (Uttam & Gunyukta), fortified ascendant & strong Moon is present, but there prevails an inauspicious omen (Shakun) then, nothing materializes regarding election.