In Jainism, Siddhi Tap refers to a special type of austerity (tap or tapa) performed to attain spiritual progress and ultimately move toward liberation (moksha) or perfection (siddhi). The word "Siddhi" means achievement, accomplishment, or spiritual perfection, and "Tap" means austerity, penance, or spiritual practice.

Siddhi Tap is observed by undertaking rigorous fasting and self-discipline. Specifically, Siddhi Tap typically consists of performing two consecutive Attham (fasting for three days and three nights without food and water) with a single meal (parna) in between. In other words:

• Three days of complete fasting (Attham)
• One meal (parna)
• Another three days of complete fasting (Attham)

This totals to six days of fasting with only one meal in between.

The purpose of Siddhi Tap is to purify the soul by shedding karmas, developing self-control, and advancing on the path toward liberation. It is considered a highly meritorious and difficult form of tapasya (austerity), practiced by spiritually advanced laypersons (shravaks) or monks.

Summary: Siddhi Tap in Jainism is a rigorous austerity involving two periods of three-day fasts (Attham) with a single meal in between, aimed at spiritual purification and progress toward liberation.