Sanskrit for "investigation, inquiry, discussion."

This represents one of the Six major schools of Hindu thought, or 'viewpoints' or darsanas. In this context it can be interpreted as meaning "the investigation of the proper interpretation of the Vedic texts." But that's a rough translation. It's primary focus being the interpretation of the Vedas as a source for ritual.

History

This didn't really start out as a school as such, but rather as a follow on from the ritual Sutra literature, whose purpose was to correctly interpret the Vedas. The problem was that these texts which described the rites and sacrifices so neccesary to ancient Indian life, were often obscure and contradictory, not to mention laced with speculation and doubts as to the meaning. It was to introduce rigour and thus authority to this informational mess that the Mimamsa was developed to provide principles which would give clear guidance as to procedure in the sacrificial rites.

The central text of Mimamsa is also the oldest one, and was written by an old sage Jaimini between 100 and 300 BC, commentaries including the tightening of definitions of important terms such as dharma were given by Sabara several hundred years later (around 500 CE). At around this time (give or take a 100 years or so) the Mimamsa school split into the subschools of the Prabhakaras and the Bhattas which are still going today. It is the only other school aside from Vedanta to have a continuity of existence from so far in the past. To give you some idea of just how old this is, Islam appeared on the scene at around 600CE, and both these schools were already present. Like Islam, they've survived to the present day.

Doctrine

The Mimamsa written by Jaimini deals with the ritual commands in the Vedas almost exclusively. It lays out the procedures and principles of the sacrifices, and the reasoning behind them. As well as this important background, it talks about "Apurva" which is a power given as reward for carrying out a sacrifice correctly, and in most cases, this power is only given to the performer once he dies. The interesting thing is that this power isn't given by Gods, but rather bestowed by the correctness of the action.

Another doctrine in the Mimamsa is that the the Vedas are eternal and uncreated, thus preclude their divine origin, and reveal the moral order of an everlasting cosmos. In this view, there's no need for a Creator God. In this respect the Hindus of the time were echoing the Ancient Greeks in viewing the world as an eternity selfsubsisting, with a population of Gods, and rituals and sacrifices to perform.