Phenomenon is singular: one observable event, fact, or remarkable person. Phenomena is plural: more than one of those. Sleepwalking is a phenomenon; lightning and tides are natural phenomena. If you can put 'a' in front of it, you want phenomenon; if you are talking about several, you want phenomena.
The core difference
Phenomenon is the singular form and phenomena is the plural, following a Greek pattern rather than the usual English -s.
- phenomenon — an observable event, fact, or occurrence, or a remarkable person or thing: Sleepwalking is a fairly common phenomenon in young children.
- phenomena — the plural of phenomenon, more than one such event or occurrence: Scientists study natural phenomena like lightning and tides.
How to tell them apart
English usually adds "-s" to make a plural, but phenomenon keeps its original Greek plural form instead, the same pattern that turns "criterion" into "criteria" and "bacterium" into "bacteria."
The mix-up shows up most often when writers treat "phenomena" as singular, writing things like "this phenomena is common." A quick check is to ask whether you mean one thing or several: one stays as phenomenon, and several become phenomena.
