According to the Mishna, a megaddef will not be held liable for his blasphemy until he clearly utters the Name of God.
According to the Rambam (Sefer HaMadda, Hilkhot Avoda Zara 2:7), uttering the Name of God means saying the four letter name of God and cursing while using one of the Names of God that cannot be erased. The Rambam quotes some opinions that believe that the four letter name referred to here is only the name that appears in the Torah that we do not pronounce – that is, pronouncing the letters yod–heh–vav–heh. The Rambam disagrees with this ruling and suggests that the four letter name is the expression of adnut – referring to God as “Lord” in the manner that we usually pronounce his name. According to the Kesef Mishna, the Rambam believes that a person who curses using either of these names would be liable for sekilah (death by stoning – see 24:16), while someone who curses one of the other Names of God would be transgressing a simple lav (biblical prohibition).
The source for the term megaddef appears in Sefer Bamidbar 15:30, although the simple meaning of the word as it appears there does not refer to cursing or blasphemy – most of the commentaries understand that that is a reference to someone who practices avoda zara – idol worship. The Sages, however, never want to make use of the term “cursing” in the context of cursing God. In fact, the Gemara often uses the expression birkat HaShem – “blessing God” – as a euphemism meaning the opposite. The Ran explains that they apply the more oblique term megaddef since when someone curses they are effectively involved in the hillul HaShem (desecration of God’s name) referred to in the pasuk.