Same entry in ps.-Zonaras (though only the second half of the quotation is given there).
The headword noun (LSJ entry at web address 1) is first attested in Thucydides 3.87.1, in reference to a comparative decline in virulence of the Plague (between its two severe outbreaks: 430-429 and 427 BCE). By the time Cassius Dio uses the word, four times, it has taken on the more limited sense of an armistice during military conflict.
cf. alpha 1913, alpha 1914 (and more generally alpha 1912); also delta 601 and pi 689.
[1] Similarly in Hesychius and other lexica.
[2] Quotation (transmitted, in Adler's view, via the Excerpta Constantini Porphyrogeniti) unidentifiable.