A casual greeting used with people you know — 'hi', 'hey'.
やあ、元気だった？久しぶりだね。
Hey, how have you been? Long time no see.
やあ、今日はいい天気だね。散歩行かない？
Hi — it's nice weather today. Want to go for a walk?
廊下で彼を見かけて、思わず『やあ』と声をかけた。
I saw him in the hallway and couldn't help saying 'hey'.
An exclamation expressing surprise or admiration — 'wow', 'oh'.
やあ、こんなところで会うなんて思わなかったよ！
Wow — I didn't expect to meet you here!
宝くじに当たったと聞いて、みんなが『やあ！』と驚いた。
When they heard about the lottery win, everyone exclaimed 'oh!' in surprise.
A shout or yell used when exerting force or effort (like 'yah!' when lifting).
二人で重い机を持ち上げるとき、『やあっ！』と掛け声を上げた。
When two people lifted the heavy desk, they shouted 'yah!' together.