1A postcard — a card sent through the mail. Often written in hiragana (はがき), though the kanji form 葉書 is also common.
年賀はがきを友人に送った。
I sent New Year's postcards to my friends.
旅行先から家族へはがきを出した。
I sent a postcard to my family from the place I traveled to.
郵便局で葉書を五枚買ってきてください。
Please buy five postcards at the post office.
2An old, now largely obsolete sense derived from 端書 (a note or postscript appended to a document). In modern Japanese はがき essentially always means a mailing postcard (sense 1); this memo/note sense is rarely if ever used today.