A Latin term meaning "from what comes before".
In legal arguments, generally means that a particular idea is taken as a given.
A priori
Jennifer hates eating vegetables. Although she really likes the smell of tomatoes, she has always believed, a priori, that tomatoes are vegetables and thus has never tried them. If Jennifer ever discovers that scientists classify tomatoes as fruit, she might very well change her mind and eat a tomato.
During the course of a trial, Lawyer Ricky has repeatedly angered the judge by arriving late and offering silly excuses for his tardiness. Today, Lawyer Ricky has a good excuse: he was the victim of a mugging on his way to work and is recovering in the hospital. However, since nobody notified the court, the judge has concluded, a priori, that Lawyer Ricky has once again neglected his responsibility to show up on time and will offer up a laughable excuse as soon as he comes in.
"If a State decides to create single-sex programs, the State would, I expect, consider the public's interest and demand in designing curricula. And rightfully so. But the State should avoid assuming demand based on stereotypes; it must not assume a priori, without evidence, that there would be no interest in a women's school of civil engineering, or in a men's school of nursing."