Augustine says (De Civ. Dei x): "The visible
sacrifice is the sacrament, i.e. the sacred sign, of the invisible
sacrifice."

_I answer that,_ All things that are ordained to one, even in
different ways, can be denominated from it: thus, from health which
is in an animal, not only is the animal said to be healthy through
being the subject of health: but medicine also is said to be healthy
through producing health; diet through preserving it; and urine,
through being a sign of health. Consequently, a thing may be called a
"sacrament," either from having a certain hidden sanctity, and in
this sense a sacrament is a "sacred secret"; or from having some
relationship to this sanctity, which relationship may be that of a
cause, or of a sign or of any other relation. But now we are speaking
of sacraments in a special sense, as implying the habitude of sign:
and in this way a sacrament is a kind of sign.

Reply Obj. 1: Because medicine is an efficient cause of health,
consequently whatever things are denominated from medicine are to be
referred to some first active cause: so that a medicament implies a
certain causality. But sanctity from which a sacrament is
denominated, is not there taken as an efficient cause, but rather as
a formal or a final cause. Therefore it does not follow that a
sacrament need always imply causality.

Reply Obj. 2: This argument considers sacrament in the sense of a
"sacred secret." Now not only God's but also the king's, secret, is
said to be sacred and to be a sacrament: because according to the
ancients, whatever it was unlawful to lay violent hands on was said
to be holy or sacrosanct, such as the city walls, and persons of high
rank. Consequently those secrets, whether Divine or human, which it
is unlawful to violate by making them known to anybody whatever, are
called "sacred secrets or sacraments."

Reply Obj. 3: Even an oath has a certain relation to sacred things,
in so far as it consists in calling a sacred thing to witness. And in
this sense it is called a sacrament: not in the sense in which we
speak of sacraments now; the word "sacrament" being thus used not
equivocally but analogically, i.e. by reason of a different relation
to the one thing, viz. something sacred.
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SECOND ARTICLE [III, Q. 60, Art. 2]

Whether Every Sign of a Holy Thing Is a Sacrament?

Objection 1: It seems that not every sign of a sacred thing is a
sacrament. For all sensible creatures