Truth (Skt. सत्य, Satya) is a categorisation of Experiences on the basis of chosen criteria.
The categories are:
Other terms for these categories are : Real and Unreal or Fact and Lie etc.
This process of categorisation occurs in Memory, specifically in the layer of intellect.
The ability to classify in this way is very natural, it happens automatically, but in a haphazard manner. A person is usually not naturally pre-equipped to do such a categorisation, it is an ability that must be learnt. However, some form of this ability is found in babies and higher animals too.
One must judge the truth based on a suitable criterion. See Criteria For Truth