noun
postal savings; postal deposit
Refers to the savings system operated by the post office, historically a major savings vehicle in Japan.
祖母は郵便貯金に毎月少しずつお金を預けている。
My grandmother deposits a little money into her postal savings account every month.
銀行預金 is a bank deposit, while 郵便貯金 is specifically a postal savings deposit.
Compound of 郵便 (postal) and 貯金 (savings).