What is the meaning of 陰平?

(調, ) dark level tone, a tone in various modern Chinese varieties, generally corresponding to the level tone and voiceless onset in Middle Chinese, e.g. tone 1 (/˥/) in Standard Mandarin (marked in Hanyu Pinyin with a macron [ ¯ ] and usually left unmarked in bopomofo), tone 1 (/˥/) in Guangzhou Cantonese, tone 1 (/˦/) in Meixian Hakka, tone 1 (/˦/) in Xiamen/Taiwanese Hokkien (unmarked in Pe̍h-ōe-jī)

Source: wiktionary.org