The "mother of Buddhas" with her great snow-white (radiant) umbrella, emblem of her protection of all beings; there are two dhāranī-sūtras that bear this name and give her description, the 佛頂傘蓋佛母 and 佛說傘蓋佛母總持陀羅尼經. 

A Dictionary of Chinese Buddhist Terms, William Edward Soothill and Lewis Hodous

（菩薩）此佛母尊有大威力，放光明，表覆蓋一切眾生，以大白傘為三昧耶形，故名大白傘蓋佛母。佛母者，諸佛之母之義也。大白傘蓋總持陀羅尼經曰：「於其空中華月輪上想白色唵字唵字放光，其光復迴，字種變成白傘金柄；柄上嚴唵字，其字放光復迴，字種變成白傘蓋佛母。一面二臂具三目，金剛跏趺而坐，右手作無怖畏印，左手執白傘當胸，嚴飾種種瓔珞，身色潔白，如雪山上日光明照，具喜悅相，顯無自性，應觀如鏡中像然。」

丁福保《佛學大辭典》