Lightning, with no e in the middle, is the electrical flash in a storm. Lightening, with the e, is the present participle of lighten — making something lighter in weight, colour or mood. The e is the whole difference and it marks the verb.
The core difference
One is a noun naming a weather event; the other is a verb form meaning "making lighter".
- lightning — the flash: struck by lightning, lightning storm. Also an adjective meaning very fast: lightning reflexes.
- lightening — from lighten: lightening the load, lightening her hair, the sky was lightening.
Why the spelling dropped its e
Lightning was once written lightening — it is historically the same word, from the flash of light. The spellings separated so the weather sense could be distinguished from the ordinary verb, which is a rare case of English tidying itself deliberately.
The test
Substitute "making lighter". If it fits, keep the e. If you mean the thing in the sky, drop it.
Frequently asked questions
Which spelling is the weather?
Lightning, with no e in the middle. Lightening with the e means making something lighter.
Is "lightening fast" correct?
No. It should be lightning fast — the comparison is to the flash, not to making something lighter.
Were they ever the same word?
Yes. Lightning was once spelled lightening; the two forms separated so the weather sense could be told apart from the verb.