अथ

Pali

Alternative scripts

अथ (atha)

Devanagari script form of atha (“moreover”)
- c. 500 AD, Kaccāyana, Pālivyākaraṇaṃ [Pali Grammar]‎^[1] (overall work in Pali), page 250; republished as Satish Chandra Acharyya Vidyabhusana, editor, Kaccayana's Pali Grammar (edited in Devanagari character and translated into English), Calcutta, Bengal: Mahabodhi Society, 1901:
  अथ सब्बासं विभत्तीनं यानि यानि पुब्बानि छ पदानि तानि तानि परस्सपदसञ्ञानि होन्ति।
  Atha sabbāsaṃ vibhattīnaṃ yāni yāni pubbāni cha padāni, tāni tāni parassapadasaññāni honti.
  Then whatever are the first six endings of all the endings, they are called the active endings.

From Proto-Indo-Iranian *átHa (“but, come!, in that case, then”), from Proto-Indo-European *áth₂o/*áth₂u (“but in addition”), a compound of the particles *át (“but”) +‎ *h₂o (“beside”), cognate with Avestan 𐬀𐬚𐬀 (aθa), 𐬀𐬚𐬁 (aθā), Hittite addu (“but, furthermore”), Latin at (“but”), and maybe Celtiberian ata (“but, furthermore”).^[1]

अथ • (átha)

a particle used at the beginning of a passage
then; afterwards; in that case
- c. 400 CE, Kālidāsa, Abhijñānaśākuntalam Act 7:
  न चेन् मुनिकुमारो ऽयम् अथ को ऽस्य व्यपदेशः ।
  na cen munikumāro ʼyam atha ko ʼsya vyapadeśaḥ.
  If this one is not a son of the muni (sage), then what family is his?
if; supposing
while; but; on the other hand
अथ चेत् ― atha cet ― but if
likewise

As the initial particle starting a passage, it conveys a sense of auspiciousness. This is often translated as now, here, etc.
It can be used as a correlative of यदि (yadi) or चेत् (cet) (both meaning "if"), corresponding to the "then" in an "if […] then" construct.
It can be used with वा (vā, “or”); अथवा […] अथवा means "either […] or".
Other diverse usages can be found in the further reading listed below.

^ Dunkel, George E. (2014) Lexikon der indogermanischen Partikeln und Pronominalstämme [Lexicon of Indo-European Particles and Pronominal Stems] (Indogermanische Bibliothek. 2. Reihe: Wörterbücher) (in German), volume 2: Lexikon, Heidelberg: Universitätsverlag Winter GmbH Heidelberg, →ISBN, page 88