Robert Mueller. REUTERS/Yuri Gripas The Justice Department appointed former FBI director Robert Mueller as special counsel to look into the ties between Russia and Trump campaign officials on Wednesday.

As calls from lawmakers and the public for an independent investigation have surged in recent months, the terms special counsel, special prosecutor, and independent counsel have been thrown interchangeably.

And while all three refer to a person tasked to investigate potential legal breaches amid conflicts of interest in the government, the three terms have been used at different points in time and for different reasons.

A special counsel is a government prosecutor who has been appointed to oversee a criminal investigation where the Justice Department has a conflict of interest or under certain circumstances where it would be "in the public interest" to appoint a investigator outside the department.

In the case of the Trump/Russia investigation, Attorney General Jeff Sessions' recusal, as well the heightened scrutiny after the firing of FBI director James Comey, prompted Deputy Attorney General to appoint Mueller as special counsel. He cited the "unique circumstances of this matter."

The appointment of a special counsel by the attorney general or deputy attorney general is "unreviewable," according to the Center for Legal and Economic Studies.

The term special counsel simply replaced the term "special prosecutor" that was used through the 1980s, after which the laws around special prosecutors expired and were not renewed, therefore retiring the term.

Both terms refer to an independent overseer who can investigate governmental records and press criminal charges.

The term independent counsel, comes from Congress's post-Watergate decision to authorize the creation of an independent special counsel for investigations in 1978 with the Ethics in Government Act. The law dictated that a three-judge panel of the US Court of Appeals in Washington, DC, appoint the counsel.

The law, which was reauthorized several times until 1999, was used more than a dozen times to initiate investigations, according to PBS Frontline. It was used perhaps most famously in the 1990s to appoint Kenneth Starr to oversee investigations into President Bill Clinton's conduct.

The current term used is "special counsel" and can be appointed only by the president, the attorney general or the deputy attorney general.

As special counsel, Mueller, once he is finished with the Trump-Russia investigation, can either pass his findings to Congress or leave the decision on what to do with the findings with the attorney general.