二

Collections of the Twenty-One Eras; twenty-one collections of waka compiled by imperial command (early Heian to early Muromachi)

twentieth day of the first month (end of New Year celebrations, upon which remaining New Year's food is eaten, decorations are stowed away, etc.)

Computing

binary tree

hook turn

Chemistry

silicon dioxide; silica

Shinto

two bows, two claps, and a third bow (common form of worshipping at a shrine)

double-dealer; timeserver; moving back and forth between two sides in a conflict

two-part work

Grammar

second-person pronoun

Music

half note; minim

Military

airman third class (JASDF)

Military

first lieutenant (JGSDF)

polarisation

Permian period

Nikai faction (of the Liberal Democratic Party); Shisuikai

every third

two ideas

double standard

Anatomy

bicuspid valve; mitral valve

half; cutting in two

Computing

binary search tree

quadratic curve

Buddhism

the road to paradise is a white road between two rivers, one of water (wrath) and one of fire (greed)

doing twice the work; duplication of work