noun
growing together; developing together
Refers to two or more things (often plants, but also abstract concepts) growing or developing in tandem. Somewhat literary or poetic.
この庭の松と梅は相生いの関係にある。
The pine and plum trees in this garden have a relationship of growing together.
二人の友情は相生いのように深まっていった。
Their friendship deepened as if growing together.
共生 is a more common and scientific term for symbiosis or coexistence, while 相生い is a rarer, more literary word emphasizing mutual growth.
Compound of 相 (ai, 'mutual') and 生い (oi, 'growth'). The exact historical derivation is uncertain, but the word is conventionally associated with the idea of things growing together.