What's that CS "big word" term for the same action always having the same effect

There's a computer science term for this that escapes my head, one of those words that ends with "-icity".

It means something like a given action will always produce the same result, IE there won't be any hysteresis, or the action will not alter the functioning of the system...

Ring a bell, anyone? Thanks.

_{Apologies for the tagging, I'm only tagging it Java b/c I learned about this in a Java class back in school and I figure that crowd tends to have more CS background...}