Today’s Word Nerd is a little tricky, since the two words are related and the difference between them is fairly subtle. Both words have a noun and verb form, multiple shades of meaning and usage, and are sometimes almost synonymous.
Affect: to cause a change
Effect: the result of a change, or the ability to cause a change
See how tricky that is? Something that affects you can produce an effect, and the effect of that is to affect you.
The “Butterfly Effect” means that small changes in the past affect the future and have a dramatic effect.
Affect, as a noun, is used in psychology to refer to feeling or emotion; a flat affect means there is no emotional response. As a verb, it means to produce an effect, or to pretend to do so.
The tornado will affect several trailer parks.
The Trekkie tried to affect an attitude of nonchalance when meeting Leonard Nimoy.
Effect, as a noun, means the consequences of a change, or the ability to affect something. The verb form means to produce a result.
The alien attack had a devastating effect.
The robot army will effect a global takeover.
Both words originated around the same time, place and language; somewhere around 1350–1400, as Middle English words with Latin roots. Affect comes from affectus, meaning acted upon or subjected to; effect is from effectus, the carrying out (of a task, job or mission, for example), and by extension, that which is carried out, or the outcome.
If you say “the tornado effected several trailer parks,” you’re saying the tornado caused the creation of several trailer parks.
If you say “the robot army affected a global takeover,” you’re saying the robot army influenced the outcome of a global takeover in some way.