A predeterminer is a grammar word that sits in front of a determiner. Determiners are words like the, a, this, that, my, his, some. In all the boys, the is the determiner and all is the predeterminer. In both his parents, both is the predeterminer. Other common ones are half (half the cake), twice (twice the amount), such (such a fool), and what (what a day). Though actually, this is teacher language. Native speakers use these words all day without naming the slot. You need the label mainly in grammar class. It is not related in meaning to predetermined (“already decided”), even though the two look like cousins. One is a place in the sentence; the other is a time-of-deciding idea.

前限定詞（文法用語：用在 the、his、that 等限定詞前面的詞，如 all、both、half）。