What does an S with a line over it mean?

An "a" with a line over it is the phonetic symbol for the sound of a long "a", as in play.

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In a prescription it means "before". Your Doctor might write a Rx like.....ibuprofen 100mg PO QD BID a.c. This means to take ibuprofen 100mg by mouth everyday twice daily before meals, and this is just a silly example, so don't take it seriously.

The prescription could have the "a.c." or just the "a" with a line over it, but that's pretty much what it stands for.

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Certain languages may put accent marks over an "a", as part of how they spell their Words.