Lakshana is a definition based on the characteristics of something. Lakshana is one of the basic criteria of any scientific treatise, according to Hindu tradition. Lakshana is defined as that characteristic which resides in a substance and nowhere else (Vatsyayana Bhashya I.1.2). Thus, the definition identifies the characteristic devoid of three faults – ativyapti (over pervasion), avyapti (non pervasion) and asambhava (improbability).
Over-pervasion means the characteristic resides in other objects, too, e,g., goh, sringitvam (the horns of a cow). This cannot be a characteristic only of a cow, because other animals also have horns. Non-pervasion means the definition applies only to some instances of that substance, e.g., the cow is red. Here, black or white cows are excluded. When the character mentioned in the definition cannot be found in the substance, it is a fault called ‘improbability’. For example, the cow is an animal that has non-cloven hooves. Actually, the cow has no such hoof at all. So this definition fails to convey the idea of the cow.
The third feature of lakshana is that it is a well known characteristic which is devoid of differences like being present in some species not is some others, and which resides in the defined object.
This definition is of two types, viz., tatastha and svarupa. Tatastha is the characteristic which neither exists till the end of the substance, nor becomes one with the nature of the substance, but still differentiates the defined object from others, e.g., that is the house of Deodatta where the crow is sitting. Svarupa is the characteristic which is the nature of the substance and differentiates the substance from others and along resides in the substance until the end of it; e.g., ‘Brahman is one who has the longing to know existence, consciousness and bliss as his innate nature.’