atto-
"atto" is a 4-letter English headword indexed on PlainSpell.
The verdict
“atto-” is outside the top-ranked English vocabulary, used as a prefix - the kind of word writers most often double-check.
- Unranked
- below top-frequency English
- 5
- letters
According to Wiktionary data (CC BY-SA, analyzed May 6, 2026) - In the International System of Units and other metric systems of units, multiplying the unit to which it is attached by 10⁻¹⁸ (short scale quintillionth or long scale trillionth).
| Property | Value |
|---|---|
| Headword | atto- |
| Language | English |
| Part of speech | Prefix |
| Letters | 5 |
| Misspellings tracked | 0 |
| Confusable pairs | 0 |
| Source | Wiktionary (kaikki.org) |
Where “atto-” sits in English frequency
Spelling & Dictionary Insight
The English entry for atto- is 5 letters long, classified as a prefix. It sits outside the most-frequent rank tiers, which is often why uncommon words generate more spelling variants per reader. The dominant gloss from Wiktionary reads: "In the International System of Units and other metric systems of units, multiplying the unit to which it is attached by 10⁻¹⁸ (short scale quintillionth or long scale trillionth).".
We couldn't generate a plausible misspelling set for atto-, a sign its spelling follows regular English conventions. We don't track a confusable pairing for this entry, which usually means its spelling is distinct enough that readers don't reach for a similar-looking word instead.
Etymologically, the entry records: From Danish atten (“eighteen”), with reference to the (negative) exponent in the relevant power of ten; the same principle also explains femto-. The correct English form is atto-, spelled A-T-T-O--.
Definition
- 1In the International System of Units and other metric systems of units, multiplying the unit to which it is attached by 10⁻¹⁸ (short scale quintillionth or long scale trillionth).
Etymology
From Danish atten (“eighteen”), with reference to the (negative) exponent in the relevant power of ten; the same principle also explains femto-.
Antonyms
This word in other languages
Definitions, pronunciation, and etymology for this entry are drawn from Wiktionary via the kaikki.org structured extract (CC BY-SA). See the methodology for how each field is sourced and updated.
Frequently Asked Questions
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Using “atto-”
The practical upshot for anyone who landed here from a spell-check.
- The one correct English spelling is A-T-T-O-- - every other letter order is a misspelling in standard orthography.
- Browse more English words and confusable pairs in the same reference. English words
Data Source
Wiktionary (via kaikki.org), licensed under CC BY-SA & GFDL. Word ordering uses an open word-frequency list; misspelling variants are generated by edit-distance from the correct headword.