invisible ink

ink which, when used for writing, cannot be seen until specially treated, eg by heat 隐形墨水（需经特殊处理方可显现字迹, 如加热）.

inveterately, invidious, invidiously, invidiousness, invigilate, invigilation, invigilator, invigorate, invigorating, invigoratingly, invigoration, invincibility, invincible, invincibly, inviolability, inviolable, inviolably, inviolate, invisibility, invisible, invisible mending, invisibly, invitation, invite, invite sb. to (sth.,a place), inviting, invitingly, invocation, invoice, invoke, involuntarily, involuntariness, involuntary, involute, involuted, involution, involve, involved, involvement, involvement in,