adverb taking the 'to' particle, taru-adjective
gushingly; welling up freely; copiously
Rare literary word describing something that wells up or gushes forth abundantly, often used with と. The reading ゆぜん is obsolete.
泉の水が油然と湧き出る。
Water gushes forth copiously from the spring.
彼の胸には感謝の念が油然として湧き上がった。
A feeling of gratitude welled up freely in his heart.
The exact derivation is uncertain; the kanji 油 (oil) and 然 (like, in the manner of) combine to suggest something flowing like oil, i.e., smoothly and abundantly. The word is a classical Chinese-derived expression (kango) now rare in modern Japanese.