Prefix (noun)

1. A syllable, word, or group of words added to the beginning of a word or base to produce a derivative word with a new meaning.
2. An element placed before another to modify or qualify its meaning.

Origin:

Late 16th century (in the sense "affix placed before a word"): from french préfixe or latin praefixum, from praefigere "fasten in front".

Examples:

1. The prefix 'un-' in the word 'unhappy' means 'not'.
2. The prefix 'dis-' in the word 'dislike' means 'opposite of or reversal of'.
3. The prefix 're-' in the word 'reheat' means 'again'.
4. The prefix 'pre-' in the word 'preview' means 'before'.
5. The prefix 'post-' in the word 'postgraduate' means 'after'.