* The most common word: In that case, it would likely be something very basic like "hola" (hello) or "gracias" (thank you).
* A word that's not food-related, but most people would understand: This could be anything from basic greetings and polite phrases ("por favor" - please, "de nada" - you're welcome) to common objects ("mesa" - table, "silla" - chair).
* A word that's unique to Spain: This would likely be a regional term or slang.
To give you a more specific answer, I need more information. What are you trying to find out?