Yaqeen means to be 100% sure and Itminan means to have a very high degree of satisfaction (around 98 to 100% confidence of being satisfied). In many situations, Islamic sharia allows Yaqeen or Itminan as a sufficient condition to prove the truthfulness of something, and obtaining of witnesses is not necessary.

This concept of Yaqeen and Itminan was first introduced in fiqh by the great shia Mujtahid, Sheikh Murtaza Ansari (who died in 1284 Al Hijri. This is supported by Quran and Sunnah. Before the time of Sheikh Ansari, witnesses were a necessary condition in fiqh to prove the truthfulness of something. Hence all Mujtahids of our time now say that to prove the truthfulness of something in many cases, it is not necessary to get witnesses, and Yaqeen or Itminan are enough.

However, in sharia, witnesses are more important than Yaqeen or Itminan in most of the cases. So if required level of witnesses do testify the truthfulness of something, then we have to follow the decision based on witnesses, and decision by yaqeen or itminan will not be followed.

Some examples of situations where Yaqeen or Itminan are enough, and witnesses may not be necessary:
(a) Deciding about the direction of qibla for salat or other things
(b) Deciding whether salat tme has started
(c) Deciding if Iftar time has come
(d) Deciding about moon sighting
(e) Deciding if a meat is halal (zabiha)
(f) Deciding if a thing is najis or not
(g) Deciding if a person is adil to lead jamat prayer

This is an individual matter of decision. A person can get Yaqeen or Itminan about a thing by getting any reliable information from any reliable source and then deciding honestly for the sake of Allah. The source of information can be anything or any sane person (including non-Muslim), which gives him yaqeen or itminan.
Based on Yaqeen or Itminan, it is thus possible, for example, that a meat is halal for one person, and the same meat is haram for another person. But if required level of witnesses testify that the meat is halal, then it is halal for both persons, even if they have differences in Yaqeen or Itminan.

Some Examples:
(a) If you have Itminan by looking at the picture of a fish that the fish has visible scales, then you can eat that fish.
(b) If a kafir says that the meat is halal, and you also attain Itminan, then you can take that meat as halal (but his statement alone is not enough).

Imam of Jamat Prayer: If you are offering salat of jamat, then either witnesses must testify or you must have Itminan that the imam leading the salat is adil. In many situations (eg when you are visiting a new place or a new mosque), you do not even know who the imam of jamat prayer is. How to get Itminan about his adalat ? This is again an individual matter. If you are satisfied about his adalat by any means (eg by his reputation, his dress, his personality, the place of salat, or by the fact that other momineen are offering salat behind him, etc), then you can consider him as adil and pray jamat behind him. But if you get neither Itminan nor witnesses about his adalat, then you can not pray jamat.

(a) Witnesses (according to standards specified by your Marja)
(b) Itminan (very high degree of satisfaction)
(c) Sahibe Yadd (owner or controller of something)

The rules of Witnesses and Itminan for proving najasat have been discussed previously in detail.

Rules of Sahibe Yadd are as follows:

Sahibe Yadd is the person or organization who is the owner or controller of something. If a Sahibe Yadd (Muslim or non-Muslim) declares that a thing under his control is najis or contains a najis thing, then you have to take it as najis (even if you may not have Itminan).
Examples of cases based on Sahibe Yadd:

(a) If the owner of a cheese making company says that it contains ingredients made from pig, then you have to believe his statement and you can not eat that cheese.
(b) If a food company writes on its food packet that it contains animal fat, then you have to believe in it. But if a third person (who is not sahibe yadd) tells you that a food contains a najis or haram thing, then the rule of Itminan will apply.
(c) If the owner or person in charge of a Chicken & Chips Takeaway shop says that the chicken is haram, then you have to believe in it. (But if he says that the chicken is halal (zabiha), then this mas’ala needs more explanation and will be discussed in next Fiqh Lectures).