有効
ゆうこう · yuukou · JLPT N3 · na-adj
valid
🎫✅ 有効 = have (有) + effect (効): it has effect, so it's valid
有 means to have or exist, and 効 means effect or efficacy (as in 効果, effect). Something that has an effect counts, so it is valid. It also sounds like you could: if a ticket is 有効, you could still use it. Its opposite is 無効 (no effect), meaning invalid.
有 — hand 𠂇 + moon/meat 月
Originally depicted a hand holding a piece of meat (月 here is the 'meat' radical, not moon), expressing the idea of possessing something. Over time the meaning broadened from 'to hold/possess' to the general sense of 'to have' or 'there is/exists'.