noun
across-the-board decline in prices; broad market sell-off
Used in financial contexts to describe a situation where prices of most stocks, commodities, or assets fall simultaneously.
今日の東京市場は全面安となった。
The Tokyo market saw an across-the-board decline today.
全面高 means an across-the-board rise in prices, the opposite of 全面安.
Compound of 全面 (zenmen, 'all fronts, overall') and 安 (yasu, 'cheap, low price').