The diacritical mark ( ៌), which indicates an orthographic ‘r’ (repha) in some words borrowed from Sanskrit (in Devanagari, repha appears as the eyelash mark above र्क, or rka). It originally represented an /r/ sound that preceded the base consonant, but in Khmer, in most cases, the consonant above which it appears, and the diacritic itself, are not pronounced. Instead, an "oa" sound is produced.
ធម៌(thɔə) = dharma (pronounced like "thoa", whereas ធ by itself would have been pronounced "tho", and ធម would have been pronounced "thom")