What is Celtic word for peace?

There isn't a single Celtic word for "peace" that encompasses all Celtic languages. Each language has its own unique word for peace, and the exact word would depend on which Celtic language you're interested in. Here are some examples:

Irish:

* Síocháin (pronounced "shee-hawn") is the most common word for "peace"

Scottish Gaelic:

* Sìth (pronounced "shee")

Welsh:

* Heddwch (pronounced "hed-vuh")

Breton:

* Peoc'h (pronounced "pwok")

Cornish:

* Peas (pronounced "pees")

Manx:

* Paase (pronounced "paase")

These are just a few examples, and the nuances of each language may offer other related words or expressions for peace. If you are interested in a specific Celtic language, it's best to consult a dictionary or grammar resource specific to that language.