要因
よういん · youin · JLPT N1 · noun
factor
要 (want/need) + 因 (cause) = the key cause you need to pay attention to
要 means 'important' or 'essential' and 因 means 'cause', so 要因 is the important cause: the one factor that matters most. Sounds like 'yoh-in', close to English 'yo-in'.
要 — west 覀 (top) + woman 女 (bottom)
Originally a pictograph of a woman with hands on her waist, meaning 'waist' (the original form of 腰). The top 覀 is a stylized depiction of the waist area, not the character for 'west'. Over time it was borrowed for the meaning 'to want, important', so 腰 (with 月 flesh radical) was created for the original 'waist' sense.
因 — enclosure 囗 + big 大
因 originally pictured a person (大) lying on a mat or cushion, shown by the surrounding box (囗). The original sense was 'mat' or 'to rely on,' which extended to 'cause,' 'reason,' and 'because of.' The 'mat' meaning was later written with 茵.