Meaning

noun (botany) a plant having a life cycle that normally takes two seasons from germination to death to complete; flowering biennials usually bloom and fruit in the second season

adjective having a life cycle lasting two seasons

adjective occurring every second year

originBorrowed from Latin bienni(um) (“two-year period”) [from bis, bi- (“twice”) + annus (“year”)] + -al (suffix forming adjectives). By surface analysis, bi- + -ennial.

Etymology from Wiktionary, CC BY-SA 3.0