Prefix
A prefix is a part of a word or a word, within a word. It is put at the start of another word to make a different meaning. It can also mean a number that is put in at the start to show which number something is in a group. It is a type of affix. compound nouns (in other cases also compound adjectives).
The following examples illustrate real prefixes.
Often people want to use a prefix to mean not. There are many prefixes that mean not.
- Unnatural means not natural.
 - Apolitical means not political.
 - Nonviolent means not violent.
 - Indirect means not direct
 
We also learnt the prefixes anti which means against and auto is used to mean self or own
Some examples of these prefixes:
antisocial which means non social
anti glare which means it will prevent sun and light glare on a screen
autograph which means a person's signature
autopilot which means working by itself
Other examples of prefixes:
- bicentennial (and other number related prefixes: mono, ter/tri, tetra, quadro, penta, hepta, octo, nano etc.)
 - contradiction
 - encompass
 - paratrooper
 - percent
 - periscope
 - prototype
 - polytechnical
 - react
 - semicolon
 
Scientists and doctors use prefixes in many words.
- Hepato- is a prefix that means liver. So hepatocellular means 'about liver cells'.
 - Hydro- is a prefix that means water. So Hydroelectric dams are dams that make electricity from the flow of water.
 - Pre- or Ante- are prefixes that mean before. Prenatal diagnostics is the diagnostics done before the birth of a baby.
 - Post- is a prefix that means after. Post-traumatic experience would be an experience that was made after trauma.
 
Sometimes people make up words by adding a prefix. These words are not in a dictionary. But if people use these words enough, sometimes they go into dictionaries. For example, we can make the word unsimple, which splits up into not simple. This is not a word in a dictionary.
Compare to suffix. Suffixes are letters put at the end of a word to change its meaning.