Considering that according to the Magyar nagylexikon Helytartótanács was the "most important central governing body of Hungary and the pivot of exercising the royal executive power" we have to come up with a name that will reflect both the history and the function. In the same reference material you can find the hierarchy of the council which will remind you of a Senate. The name Royal Senate would also be descriptive enough, however the name Senate may be slightly confusing because of association with other periods of history when there was a senate in power. The recommended term is easily understandable by modern-day readers as well, because it expresses the period (kingdom) and the function (council) and the composition (governors or helytartók).