I've been reading some online articles on the concept of "function", but I'm not

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I've been reading some online articles on the concept of "function", but I'm not very sure about it. An ashtray, according to my dictionary, is a "container for cigarette ash", but I don't know what this "for" means. It can't mean that people ought to put ashes in the ashtray, because there are other places where we may put it. And it can't mean that people may put there the ashes, since, once again, we may put the ashes in places which are not ashtrays. It can't either mean that the ashtray was made "with the purpose" of serving as a container for the ashes, because an object may be an ashtray now but haven't been made to be an ashtray. So, what is an ashtray?