In Jain tradition, “Aatham” (also written Atham/Aatham) usually means:

• The 8th lunar day (Ashtami) of each fortnight: A very auspicious tithi. Many Jains do upvaas (fast), ayambil, pratikraman, and extra svādhyāy on this day. It is one of the “pañch tithi” observed for stronger spiritual benefit.

• “Attham” as a tapas: A three‑day continuous fast (about 72 hours). It traditionally concludes on the Aatham tithi. There is also “Atthai” (eight‑day fast), a different and longer austerity.

Tradition notes

• Shvetāmbara and Digambara both honor Aatham (Ashtami) as a special day. Maryādā can differ a little:

- Shvetāmbara laity often take only boiled water during daylight when fasting. - Digambara practice commonly emphasizes nirjal (no water) unless guided otherwise by an ācārya.

• The aim in both is samvar (stopping new karma) and nirjarā (shedding bound karma) through right faith, restraint, and penance.

Essence

• As a tithi: Aatham = 8th day, a powerful time for vrata and dharma-kriyā.
• As a fast: Attham = three days of continuous fasting, ideally culminating on Aatham, undertaken with humility, right conduct, and proper pāraṇā.

For simple overviews of Jain fasts and important tithis, you can refer here:

• Types of Jain fasts
• The five special tithis (includes Ashtami/Aatham)