A few years ago, a lot of people started using the word "impact" in place of the word affect, as in, "How will accounting be impacted by this decision?" It seemed another attempt to inject excitement into the language of the workplace, along the lines of overcharged expressions such as "firing off an e-mail."
But the displacement of affect by impact was a poor choice, and not only because affect is a perfectly good word; it turns out that the past participle impacted is sometimes used by those in the medical profession as a synonym for "constipated."
Affect is a great word, and it only causes problems when it's confused with its cousin, effect. Conveniently, the word affect is most often used as a verb. It means to produce an effect on, to attack (as a disease would do), to move people emotionally or to pretend.
Examples of affect in context are:
The only time affect is used as a noun is in the context of psychology, meaning feeling or emotion: "Symptoms include a blandness of affect, lack of appetite and sleeplessness."
How will accounting be affected by this decision?
The cold affected his sinuses.
The audience was deeply affected by the film.
By contrast, the word effect is used primarily as a noun, meaning result, consequence, efficacy or impression. It can also mean a natural phenomenon, such as the Doppler effect. The plural form, effects, has two additional meanings: property (e.g. personal effects) or creative use of sound and light (e.g. theatrical effects).
There is a confusing verb form of effect that means "to bring about." It mostly appears when reading about someone who wishes "to effect change." Maybe one change we can effect is to encourage the use of affect rather than "impact." Who knows? It just might have a positive effect.
Sources: Oxford Dictionary of Current English, The American Heritage Dictionary of the English Language and the fine people at Park Nicollet Clinic.