noun · 3 senses · updated from the 2026-07-25 local source snapshot
Definitions and examples are grouped by meaning. Pronunciation, history, word forms, translations, descendants, synonyms, antonyms, derived terms, and related words appear whenever the source provides them.
Sound
Pronunciation
Each Play button uses your device's default local English voice, or a natural local fallback. The selected voice name appears after playback. This is synthesized speech, not a source recording.
/ˌreɪdɪəʊakˈtɪvɪti/
Southern-England:Recorded audio is listed in the source snapshot.
CMU ARPABET:R EY1 D IY0 OW0 AE2 K T IH2 V AH0 T IY2
Digits mark lexical stress: 1 primary, 2 secondary, 0 unstressed.
CMU ARPABET 2:R EY2 D IY0 OW0 AE2 K T IH1 V AH0 T IY2
Digits mark lexical stress: 1 primary, 2 secondary, 0 unstressed.
1
noun
Meaning 1
Spontaneous emission of ionizing radiation as a consequence of a nuclear reaction, or directly from the breakdown of an unstable nucleus.
Definition source: English Wiktionary via Wiktextract
Usage: uncountable, usually
No example sentence was provided for this meaning.
Meaning relationships
Synonyms: none provided
Antonyms: none provided
2
noun
Meaning 2
The radiation so emitted; including gamma rays, alpha particles, neutrons, electrons, positrons, etc.
Definition source: English Wiktionary via Wiktextract
Usage: uncountable, usually
No example sentence was provided for this meaning.
Meaning relationships
Synonyms: none provided
Antonyms: none provided
3
noun
Meaning 3
the spontaneous emission of a stream of particles or electromagnetic rays in nuclear decay
Definition source: Princeton WordNet 3.0
No example sentence was provided for this meaning.
