In Jain Dharma, we don’t eat onion mainly because of ahimsa (non‑violence) and mental purity.

Very simply:

1. Onion is a root (kand-mool)

- When you pull out an onion, you uproot the whole plant. - This kills the plant and also disturbs/kills many tiny living beings (micro-organisms, insects) in the soil. - Compared to cutting leaves or fruits, this is considered more himsa (violence). ( jainknowledge.com)

1. Infinite small lives (anantkay / bahu-jiva)

- Roots often contain or shelter countless small lives. - By eating such roots, we are seen as harming many jivas at once, which increases bad karma. ( jainknowledge.com)

1. Effect on mind and senses

- Onion (and garlic) are very strong, pungent foods. - They can excite passions, disturb calmness, and make meditation and self-control harder. - Jain Dharma encourages a simple, peaceful, sattvic diet that keeps the mind light and pure. ( jainknowledge.com)

1. Both main traditions

- Digambar and Shwetambar Jains both generally avoid onions and other root vegetables. - Strictness can vary by family or region, but the basic reason is the same: minimize himsa and keep the mind pure. ( jainknowledge.com)

If you want to read a bit more in simple language, you can see this explanation on JainKnowledge: “Why do we not eat onions and garlic?”