Numeral
いちまん
わたしのぜんざいさんは一万円です。
Ten thousand yen is all I have.
一万RTいったら、こく白します。
If this gets ten thousand retweets, I'll confess my love.
ちくしょう！にせの一万円さつをつかまされた。
Crap! I was cheated into accepting a fake 10,000 yen bill.
When you have one ten thousand you just have ten thousand. In Japanese, though, you specify how many ten thousands you have, though (which is weird in English, but should make sense if you think about it).
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.