Aambill (also spelled Ambil or Āmbil) is a special type of austerity (tapa) observed by Jains. It involves eating simple and plain food, typically only once a day, without the use of milk, curd, ghee, oil, sweets, green/raw vegetables, fruits, and spices. The food is usually boiled lentils or grains with salt and water. The purpose of aambill is to practice self-restraint, simplicity, and detachment from taste and sensual pleasures, which helps in purifying the mind and reducing passions (kashayas).

Aambill is often observed during Jain festivals such as Ayambil Oli, which is a special period (usually 9 days) dedicated to intense spiritual practices. During this observance, participants focus on prayer, meditation, and self-discipline along with the dietary restrictions.

In summary, aambill is a Jain austerity emphasizing plain food and spiritual discipline to aid in the path of self-realization and non-attachment.