はがき, 葉書, 葉書き, 端書, 端書き, 羽書, ハガキ
See 郵便葉書, Abbreviation, Usually in kana
1. postcard
2. memo, note, card
はがきにこの切手を貼って郵便箱に投函しなさい。
は
Particle, pronounced わ in modern Japanese
1. indicates sentence topic
2. indicates contrast with another option (stated or unstated)
3. adds emphasis