Pramaan has several meanings --

(1) Measurement
Pramaan means measurement, one which that measures, such as a foot, a yard stick, a weight - pond, Kilogram etc etc.
It is also a valid means of knowledge.

(2) Proof
The Darshan (six of them in Hindu philosophy) accept some of the means of knowledge upon which further theories are developed. They are the following :

(1) Pratyaksh (direct or immediate perception) - The Pratyaksh Pramaan has two stages- Nir-Vikalp Pratyaksh and Sa-Vikalp Pratyaksh. There will be a general awareness of the sense object as soon as the sense organ comes into its contact (Nir-Vikalp Pratyaksh). When the details are noted in the light of the past experience, it leads to Sa-Vikalp Pramaan.

(2) Anumaan (inference, guess) - When we see a sign connected to the original, it gives us the indirect knowledge (as in the case of a smoke inferring that there is a fire), it is Anumaan Pramaan.

(3) Upamaan (comparison) - When a Pramaan is established by comparison of the new object with the earlier seen object, Upamaan Pramaan is established.

(4) Shabd (verbal testimony) - Words of a reliable person (Aapt Vaakya) or the authority of a Vaidik injunction could be considered as Shabd Pramaan. This can be either Paurusheya or A-Paurusheya.

(5) Arthaapatti (postulation or presumption) - When an unperceived fact alone can explain an anomaly satisfactorily, a postulation or presumption becomes necessary (Arth Aapatti). This knowledge is distinctive since it cannot be got by any other means.

(6) Anupalabdhi (non perception) - Anupalabdhi or non perception gives immediate cognition of the non-existence of an object. (The object which was seen earlier is not there and so, its non-existence is recognized).

The Darshan (Philosophy) are all based on these Pramaan only.

Some say there are 8 types of Pramaan -- (1) Pratyaksh, (2) Anumaan, (3) Upamaan, (4) Shabd, (5) Aitihya, (6) Arthaapatti, (7) Sambhav and (8) Abhaav

(1) Pratyaksh - Whatever knowledge is gained through our Gyaan Indriyaan is called Pratyaksh Pramaan. Because our Indriyaan are connected with our mind and and our mind is connected with our soul.

(2) Anumaan - When something is guessed seeing something else, is called Anumaan, as seeing the smoke one can say that there is a fire. This Anumaan is of three types - (1) Poorv-vat - seeing clouds one can predict rains, seeing a married couple one can guess of children, means first see the cause and then guess of action. (2) Shesh-vat - where seeing the action the cause can be guessed - as seeing lots of water in a river one can guess heavy rains in the mountains, or seeing a punished person one can guess of his crime; (3) Saamannyato-drisht - where neither there is any cause nor there is any action, but there is some Dharm, such as without moving one cannot go anywhere.

(3) Upamaan - When something is similar to the other, it is called Upamaan. Such as if you say - "call Vishnu" the other man says - 'I have never seen him." "Oh, he is like Raam." So he went and saw Vishnu, found him like Raam and brought him together.

(4) Shabd Pramaan - Aaptopadeshah Shabdah. Who are Aapt, means are fully learned, wise, wish for others welfare their words are also Pramaan.

(5) Aitihya - Copying from somebody's life actions

(6) Arthaapatti - Such as somebody said - "rain falls from clouds", taking it as it is called Arthaapatti

(7) Sambhav - Whichever things are possible according to the world's system, those are Sambhav, otherwise they are A-Sabhav (impossible)

(8) Abhaav - If something is not there and one brings it from somewhere else, it is called Abhaav.

If one takes Aitihya in Shabd, Arthaapatti in Anumaan and Abhaav in Sambhav then only five Pramaan remain - which are given above also.