noun

• Something placed before another
  "The chosen prefixes won the vote in part because they start with the only two letters left in the alphabet that are not already used in measurement. The b for “bronto” is already used for bytes and h for “hella” is used for hecto, the prefix for 100."
• Something placed before another
  "in the UK, a number with an 0800 prefix is a toll-free number."
• Something placed before another
  "The string "abra" is both a prefix and a suffix of the string "abracadabra"."

verb

• (transitive) To determine beforehand; to set in advance.
  "But the danger was, that a man can hardly prefix any certaine limits unto his desire[…]."
• (transitive) To put or fix before, or at the beginning of something; to place at the start.