The six 外道論 vedāngas, works which are 'regarded as auxiliary to and even in some sense as part of the Veda, their object being to secure the proper pronunciation and correctness of the text and the right employment of the Mantras of sacrifice as taught in the Brāhmaṇas '. M. W. They are spoken of together as the 四皮陀六論 four Vedas and six śāstras, and the six are Siksā, Chandas, Vyākarana, Nirukta, Jyotisa, and Kalpa.

A Dictionary of Chinese Buddhist Terms, William Edward Soothill and Lewis Hodous

古代印度外道所用六種論。又作六皮陀分。與四韋陀、八論等合稱十八大經，即金七十論第二十三章所說之外智。(一)式叉論（梵 Śiksa），闡釋六十四能法。(二)毘伽羅論（梵 Vyākarana），闡釋諸音聲法。(三)柯剌波論（梵 Kalpa），闡釋諸天仙上古以來之因緣名字。(四)豎底沙論（梵 Jyotisa），闡釋天文、地理、算數等法。(五)闡陀論（梵 Chandas），闡釋作首盧迦法。(六)尼鹿多論（梵 Nirukta），闡釋造立一切物名之因緣。〔百論疏卷上之下〕

佛光電子大辭典

p1309

（名數）外道六種之論也。婆藪盤豆法師傳曰：「四皮陀六論。」百論疏上之下曰：「六論者：一式叉（S/iks!a%）論，釋六十四能法。二毘伽羅（Vya%karan!a）論，釋諸音聲法。三柯刺波（Kalpa）論，釋諸天仙上古以來因緣名字。四豎底沙（Jyotis!a）論，釋天文地理算數等法。五闡陀（Chandas）論，釋作首盧迦法，佛弟子五通仙等說偈名首盧迦。六尼鹿多（Nirukta）論，釋立一切物名因緣。」

丁福保《佛學大辭典》