同一
どういつ · douitsu · JLPT N3 · na-adj
similarity
同一 = same + one: two things that are one and the same
同 means same (as in 同じ, onaji) and 一 means one. Things that are the same one thing share an identity. Sound hook: douitsu sounds like 'though it's you', the same person all along.
同 — 冂 enclosure + 一 + 口 mouth
Originally depicted multiple mouths gathered under one cover or enclosure, suggesting people speaking together in agreement. From this image of voices joining, it came to mean 'same,' 'together,' or 'alike.' The shared cover symbolizes unity.
一 — single horizontal stroke
一 is one of the simplest pictographs in Chinese, depicting a single horizontal line to represent the number one. It has retained essentially the same form since oracle bone script over 3,000 years ago. By extension it also means 'first,' 'whole,' or 'same.'