Origin Jain fasting of restriction, kept especially in the two Ayambil Oli fortnights
A fast of restriction rather than abstention: one meal in the day, taken before dark, of plain boiled food with no taste added to it.
What is excluded defines it. There is no milk, curd, ghee, oil, sugar or jaggery, the rich foods Jain usage groups as vikriti, those that provoke, and no green vegetables, no raw food and no fruit. What remains is boiled grains and pulses, plain, eaten once, with boiled water. The food is deliberately without flavour, and this is the substance of the observance rather than an incidental consequence of the rules: the practitioner eats enough to continue and takes no pleasure from it, which addresses the appetite rather than the body's need.
That distinguishes it from complete fasting, and Jains treat the two as doing different work. Upvas suspends eating; ayambil keeps a person eating daily and removes what makes eating attractive, which can be sustained far longer. Extended ayambil observances run for many days and some Jains undertake them for very long periods.
The observance has its own festival. Ayambil Oli falls twice a year, in the spring and the autumn, and runs for nine days, during which participants take ayambil each day and worship the Navpad, the nine venerated categories. Jain temples and community halls operate ayambil kitchens, ayambil shalas, which prepare the permitted food for anyone keeping the observance, so that the difficulty of cooking it does not become a reason not to.