Domain knowledge is that valid knowledge used to refer to an area of human endeavour, an autonomous computer activity, or other specialized disciplin.
In software engineering, domain knowledge is knowledge about the environment in which the target system operates, for example, software agents. Domain knowledge is important, because it usually must be learned from software users in the domain (as domain specialists/experts), rather than from software developers. Expert's domain knowledge (frequently informal and ill structured) is transformed in computer programs and active data, for example in a set of rules in knowledge bases, by knowledge engineers.
Communicating between end-users and software developers is some how difficult. They must find a common language to communicate in. Developing enough shared vocabulary to communicate can often take a while.
The same knowledge can be included in different domain knowledge. Knowledge which may be efficient in every domain is called domain-independent knowledge, for example logics and mathematics. Operations on domain knowledge are performed by meta-knowledge.
Some Popular Domain are
Finance,Retail,Telecom,Legal,Medical,Hospital,Sports,Education etc