To decide, choose, or determine; to make up one's mind about something.
旅行の行き先を家族で決めた。
The family decided on a destination for the trip.
会議で次のプロジェクトの担当者を決めた。
We decided who will be in charge of the next project at the meeting.
彼は留学することを決めた。
He decided to study abroad.
日程は来週の金曜日に決めてください。
Please decide the schedule for next Friday.
To clinch or decide the outcome (e.g., of a match); to produce a decisive play that determines victory.
最後の一投で試合を決めた。
He decided the match with the last throw.
彼のシュートが勝敗を決めた。
His shot decided the outcome of the game.
To persist in doing something; to go through with a plan or action to completion.
一度始めたらやり抜くと決めている。
Once I start something, I've decided to see it through.
チームは最後まで勝利を信じて決めて戦った。
The team fought determinedly, believing in victory until the end.
To habitually do something; to have made a regular practice of doing something (often used as 決めている).
朝は必ず散歩すると決めている。
I always decide to take a walk in the morning (i.e., I make a habit of it).
健康のため、毎日野菜を食べると決めている。
For my health, I've decided to eat vegetables every day.
To take something for granted or assume it to be true (often used in phrases like 〜と決めてかかる).
彼はみんなが賛成すると決めてかかっていたが、実際は意見が分かれていた。
He assumed everyone would agree, but in reality opinions were divided.
そんなに簡単だと決めつけないでください。
Don't assume it's that simple.
Colloquially, to dress up or style oneself sharply (often written キメる in katakana).
今夜はパーティーだから、ちょっとキメて行こう。
There's a party tonight, so let's dress up a bit.
彼は写真のために髪と服をキメた。
He dressed up his hair and clothes for the photo.
To pull off or execute a move, pose, or action successfully (to 'nail' something in sports, dance, etc.).
彼女は最後のジャンプを完璧に決めた。
She nailed the final jump perfectly.
決めポーズを決めて写真を撮った。
We struck a signature pose and took a photo.
監督はラストプレーでエースに決めさせた。
The coach had the ace player decide the final play.
In martial arts/sumo, to immobilize an opponent (e.g., with a double-arm lock) — to apply a finishing hold.
柔道で相手に腕十字を決めた。
He applied an armbar on his opponent in judo.
Colloquially, to have a drink (e.g., 'to knock one back'); in slang, to take illegal drugs (context-dependent).
仕事帰りにビールを一杯決めた。
After work I had a beer (knocked back a beer).
スラングで『キメる』と言うと薬を使う意味になることがあるので注意が必要だ。
In slang, 'キメる' can mean to take drugs, so be careful with its use.