A prescription, as is well known, is a physician’s order for the preparation and administration of a drug or device for a patient. What may be less well known is that a prescription has several parts:

The superscription (or heading) with the symbol R or Rx which stands for the word Recipe, meaning (in Latin) to take;
The inscription which contains the names and quantities of the ingredients;
The subscription or directions for compounding the drug; and
The signature which is often preceded by the sign s. standing for signa, mark, giving the directions to be marked on the container.
There are other words on the prescription. For example, b.i.d. It means twice (two times) a day and is an abbreviation for “bis in die” which in Latin means, not too surprisingly, twice a day. It is one of a number of abbreviations of Latin terms that have been traditionally used in prescriptions to specify the frequency with which medicines should be taken.
Some of the abbreviations of terms commonly used in prescriptions with their meanings are:
a.c. = before meals (from “ante cibum,” before meals)
ad lib: use as much as one desires (from “ad libitum”)
b.i.d. = twice a day
caps = capsules
da or daw = dispense as written
g (or gm or GM) = gram
gtt. = drops (from “guttae,” drops)
h. = hour
mg = milligram
ml = milliliter
p.c. = after meals (from “post cibum,” after meals)
p.o. = by mouth, orally (from “per os,” by mouth)
p.r.n. = when necessary (from “pro re nata,” for an occasion that has arisen, as circumstances require, as needed)
q.d. = once a day (from “quaque die,” once a day)
q.i.d. = four times a day (from “quater in die,” 4 times a day)
q._h.: If a medicine is to be taken every so-many hours (from “quaque,” every and the “h” indicating the number of hours)
q.h. = every hour
q.2h. = every 2 hours
q.3h. = every 3 hours
q.4h. = every 4 hours
t.i.d. = three times a day (from “ter in die,” 3 times a day)
ut dict. = as directed (from “ut dictum,” as directed)
I find that the word “prescription” also comes from the Latin “praescriptus” and is made up of “prae,” before + “scribere,” to write, so that prescription meant “to write before.” They say that this reflected the historic fact that a prescription had traditionally to be written before a drug could be prepared and then administered to a patient.
More via fda.gov here.