Latin for “at first glance” or “on first impression”.
It is generally used to describe a starting point for discussion or an initial view about a state of affairs which may well be changed by evidence or argument to the contrary.
By way of example (highlighted in bold): Richard v British Broadcasting Corpn  EWHC 1837 (Ch);  Ch 169, per Mann J:
“ These judicial remarks demonstrate at least some of the reasons why an accused should at least prima facie have a reasonable expectation of privacy in respect of an investigation. They are particularly appropriate to the type of case referred to there (of which, of course, the present case is an instance) but they are generally applicable, to varying extents, to other types of cases.
 That is not to say, and I do not find, that there is an invariable right to privacy. There may be all sorts of reasons why, in a given case, there is no reasonable expectation of privacy, or why an original reasonable expectation is displaced. An example was given by Sir Brian Leveson in the extract quoted above, and others can be readily thought of. But in my view the legitimate expectation is the starting point. I consider that the reasonable person would objectively consider that to be the case.”