A prefix meaning 'having to do with or related to phthalic acid'. Also phthalo-.
Noded for the express purpose of informing you that 'phth' is pronounced the same as 'th'. For example, phthalic is pronounced thalik or thaylik.
Note, however, that not all phth words have to do with phthalic acid. Phthisis, for example, comes from the Greek word meaning consumption; Phthor (meaning Fluorine) comes from the Greek for 'to destroy'. They are pronounced Thises and Thor, respectively.
Phthal- also comes to us by way of Greek, although it was originally Arab, from the Arabic word naft or nift. Naft into phthal? Well, there is a step I've left out. Naft transmogrified into naphtha, meaning, more or less, petroleum. When Auguste Laurent first created phthalic acid in 1836, he thought that it was a naphthalene derivative, and so named it naphthalenic acid. Jean Charles de Marignac later showed this to be incorrect; Laurent changed the name to the modern term, 'phthalic acid'.