What is a lactone obtained by reduction of phthalyl chloride, as a white crystalline substance; hence, by extension, any one of the series of which phthalide proper is the type. [Written also phthalid.] called?

phthalide — A lactone obtained by reduction of phthalyl chloride, as a white crystalline substance; hence, by extension, any one of the series of which phthalide proper is the type. [Written also phthalid.]. (Webster's 1913.)