"Seelonce" Natural Recordings by Native Speakers
I couldn't find any words in the English language that are spelled exactly as "seelonce". However, I'm going to take a guess that you might be thinking of "leoncino" or "selence" or "onsense", but none of these words exist in the English language.
However, "once" is an English word that means "at one time; a single time; a single occasion; before".
If I had to make an educated guess, I'd say "seelonce" might be a misspelling of "essentially once" or "a once peaceful place", but I couldn't find any word or phrase in English that matches this exact spelling.
If you are referring to something else entirely, could you please provide more context or information about where you encountered this word?