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| Name formality | Formal |
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| Celestial body | Earth |
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| Regional usage | Global (ICS) |
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| Time scale(s) used | ICS Time Scale |
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| Chronological unit | Period |
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| Stratigraphic unit | System |
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| Time span formality | Formal |
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| Lower boundary definition | Defined chronometrically |
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| Lower GSSA ratified | 1990 |
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| Upper boundary definition | Redefined chronometrically with an interim calibrated age of c. 720 Ma. GSSP is in progress. (Previously defined chronometrically as 850 Ma) |
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| Upper boundary definition candidates | The first appearance of widespread glaciation. |
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| Upper boundary GSSP candidate section(s) | To be determined |
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| Upper GSSP ratified | Not formally defined |
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The Tonian (from Ancient Greek: τόνος, romanized: tónos, meaning "stretch") is the first geologic period of the NeoproterozoicEra. It lasted from 1000 to 720 Mya (million years ago). Instead of being based on stratigraphy, these dates are defined by the ICS based on radiometric chronometry. The Tonian is preceded by the Stenian Period of the Mesoproterozoic Era and followed by the Cryogenian.
