以免 (yǐmiǎn) is a formal conjunction that introduces a negative purpose clause — 'so as to avoid / in order to prevent' an undesirable outcome. Unlike 免得 (miǎnde), which is colloquial and often used in everyday speech, 以免 is more common in writing and formal contexts. It contrasts with 以便 (yǐbiàn), which introduces a positive purpose ('so that'), whereas 以免 prevents something bad from happening.