Experts as a source of Audit Evidence

Mostly the auditor's work on evidence obtained from along within the entity supported through confirmations from third parties will provide him enough reliable evidence which can enable him to provide an unqualified opinion. However there are certain circumstances where his knowledge is not enough and he may see requirement to rely on the opinion of other experts to support him from his opinion. 

The authoritative document on this subject on ISA 620: Using the work of an expert.  Hence it defines an expert or a specialist such a firm possessing special skills or a person, experience and knowledge in a particular field other than auditing and accounting.  The expert may be engaged through the client or through the auditor; he may be employed through the client or through auditor.