A Mentat is a practitioner of the mentat discipline in Frank Herbert's series of science fiction novels starting with Dune and ending with Chapterhouse Dune, six novels and eight thousand years later.
Following the Butlerian Jihad in the Dune universe's back story, it was forbidden to create machines "in the images of man's mind". Thinking machines were outlawed throughout the universe, and the mentat discipline was developed as a replacement. Mentats are humans trained to be computers: human minds developed to staggering heights of cognitive and analytic ability.
Unlike computers, however, mentats are not simply human calculators writ large. Instead, the exceptional cognitive abilities of memory and perception are the foundation for supra-logical hypothesizing. Mentats are able to sift large volumes of data and come up with concise analyses in a process that goes far beyond logical deduction: Mentats cultivate "the naive mind", the mind without preconception or prejudice, that can extract the essential patterns or logic of data and deliver, with varying degrees of certainty, useful conclusions. They are not limited to formulating syllogisms; they are the supreme counselors of the Dune universe, filling roles as menial as archivists and clerks, or as grand as advisor to the Emperor.
Being a mentat can be a profession in itself, or the discipline can be added to other roles. In Dune, Paul Atreides is a mentat duke (later emperor); in Heretics of Dune and Chapterhouse Dune, Miles Teg is a mentat bashar, or general, who is the most formidable military commander in the known universe. It's also possible to vary the discipline: In Dune, the Tleilaxu create 'twisted mentats' for the Baron Vladimir Harkonnen (Piter de Vries being the most notable example), the purpose of which is a sociopathic mentat free from the usual constraints of human morality or instinctual considerations (de Vries is, among other things, an accomplished sadist, and acts as torturer for the Baron). In Dune Messiah, Duncan Idaho is recreated as a ghola, and trained as both a mentat and as a Zensunni philosopher; he then serves Paul Atreides as the house weapons master. In Children of Dune, he marries the regent, Alia, but is then instrumental in protecting the royal heirs from her, demonstrating that mentats are rarely simply disinterested observers and counselors.
The ability to be a mentat is a talent limited to a small number of people; thus, those with the discipline are prized in whatever role they choose. There are also varying degrees of ability. In Dune, Thufir Hawat is considered to be one of the best mentats known at the time.
Mentat is the name given to a macro-dataflow extension of the C++ programming language. It was developed at the University of Virginia computer science Department by a research group led by Andrew Grimshaw. The combination of the ideas needed to implement the Mentat run-time with the ideas in CMU's Hydra distributed operating system led to the Legion distributed OS.