British or sulfur (VI) oxide US (ˈsʌlfə sɪks ˈɒksaɪd)
noun
another name for sulphur trioxide
sulphur trioxide in British English
noun
a white corrosive substance existing in three crystalline forms of which the stable (alpha-) form is usually obtained as silky needles. It is produced by the oxidation of sulphur dioxide, and is used in the sulphonation of organic compounds. Formula: SO3