noun
one yen
Refers to the one-yen coin or the amount of one yen. The one-yen coin is the smallest denomination of Japanese currency.
一円玉は軽くて、財布の中でよく迷子になる。
The one-yen coin is light and often gets lost in my wallet.
消費税のせいで、一円単位の
Because of the consumption tax, payments in one-yen increments have increased.
suffix, adverb
throughout; the whole area
Used as a suffix or adverb to mean 'throughout an area' or 'the whole district'. Often follows a place name or area-denoting noun.
関東一円に大雨警報が出ている。
A heavy rain warning has been issued throughout the Kanto region.
この辺り一円は、昔は田んぼだったそうだ。
I heard that this whole area used to be rice fields.
全域 is a more formal, straightforward noun meaning 'the whole area', while 一円 has a slightly more literary or set-phrase feel when used as a suffix/adverb.
Compound of 一 (one) and 円 (circle, yen). The 'throughout' sense likely derives from the idea of a complete circle encompassing an area.