壹
イチ, イツ, ひとつ, い

number one

JLPT0 G13 S0 F5000

The relevant seal inscription form is 壺 lidded pot + 吉# (filled container) → full pot/jar → *one* (← unity of contents). As with 弐, 参/參, and 拾, 壱 is used as a substitute for the usual number (in this case, 一) in documents as a precaution against forgery.

[Full Kanji Etymology List]