twice a day / twice daily / 2 times daily
a prescription, b.i.d. means twice (two times) a day. It is an abbreviation for "bis in die" which in Latin means twice a day. The abbreviation b.i.d. is sometimes written without a period either in lower-case letters as "bid" or in capital letters as "BID"
q.d. (qd or QD) is once a day; q.d. stands for "quaque die" (which means, in Latin, once a day). t.i.d. (or tid or TID) is three times a day ; t.i.d. stands for "ter in die" (in Latin, 3 times a day).
q.i.d. (or qid or QID) is four times a day; q.i.d. stands for "quater in die" (in Latin, 4 times a day).
q_h: If a medicine is to be taken every so-many hours, it is written "q_h"; the "q" standing for "quaque" and the "h" indicating the number of hours. So, for example, "2 caps q4h" means "Take 2 capsules every 4 hours."