Pachakhan (also written Pachkhan / Pacchakkhan) in Jainism means:

• A firm vow or pledge you take, usually in front of Bhagwan, a guru, or facing the temple.
• You clearly decide and state what discipline you will follow – mostly about food and drink (like when you will eat, when you will stop, what you will avoid).
• It is taken for a fixed time (for example, for that day, or for a few days).

In simple words: Pachakhan = “I promise, from now till this time, I will follow this specific niyam (rule).”

Common examples:

• Navkarsi Pachakhan – I will not eat or drink till a short time after sunrise.
• Porsi / Sadhporsi Pachakhan – I will eat even later after sunrise.
• Panhar / Chauvihar Pachakhan – I will not eat (and sometimes not drink) after sunset.

Why we do Pachakhan:

• To practice self-control and reduce desires.
• To support tapas (austerity) and nirjara (shedding of karmas).
• To make our daily routine more disciplined and spiritual.

You can read a simple explanation here