An omen is a phenomenon that is believed to foretell the future, often signifying the advent of change. Though the word 'omen' is usually devoid or 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'.
In the field of astrology, solar and lunar eclipses have often been considered omens of notable births and deaths.
Omens may be considered either good or bad depending on their interpretation. The same sign may be interpreted differently by different people or different culture.
The treatises on Hindu astrology have discussed omens in detail in regards to travel elections.
Good Shakunas in Vedic Astrology:
Fresh meat, liquor, Burning fire, white flower, Ghee, sandal, Akshatham (Paddy, rice and flower), Curdled milk, plantain fruit, two Brahminns, Elephant, prostitute, honey, King, Sugar cane, Cow-bull with coir etc.
Ashes, firewood, Oil, Donkey, broomstick, Muram (a sieve to winnow grain), Darbha (the grass Poa Cynosuroides used in Hindu oblations, rites etc.), Salt, widow, Patient, snake, Iron, cat, flowers used in funeral, Handicapped, gingelly seed, a call from back when you start your journey, etc.
On seeing an inauspicious omen the treatises state the person should halt their journey and return to the starting point. Up on reaching the starting point the traveler is advised to recite Pranayamam(a specific mantra's recitation) eleven times and then start the journey ones more. If an inauspicious omen is again seen during the trip the traveler should return to the starting point once more and recite Pranayamam sixteen times, restarting the journey once more. Should an inauspicious omen be observed a third time the treatises state the journey should be abandoned.
In Nimitha numerous different aspects of the omen come in to play in interpreting what the omen means. The severity of an omen is assessed based on its position with respect to the observer, its direction in respect to the observer, the time of its observation, the speed of the omen, the sounds heard during the omen,and the place where an omen is observed.