Doing something together (in the same place/time or jointly); often used adverbially with ～に.
毎週末、家族と一緒に公園へ行きます。
Every weekend I go to the park with my family.
コンサートでみんなで一緒に歌った。
At the concert we all sang together.
宿題は友達と一緒にやった方が早い。
It's faster to do homework together with a friend.
Putting things together as one lump or mixing items; used in senses like 'to lump together' or 'to mix'.
彼の言動を過去の失敗と一緒にして評価してはいけない。
You shouldn't judge his actions together with his past mistakes (i.e., lump them together).
材料をボウルに入れて一緒に混ぜる。
Put the ingredients in a bowl and mix them together.
可燃ゴミと不燃ゴミを一緒に出してはいけない。
You must not put combustible and non-combustible trash out together.
Being the same; identical (used in expressions like 'the same' or 'identical').
彼と私は一緒の制服を着ている。
He and I are wearing the same uniform.
この二つの結果は一緒だ。
These two results are the same.
鍵を彼のと一緒にしてしまったかもしれない。
I might have put my key together with his (i.e., mixed them up).
Happening at the same time; occurring simultaneously (used adverbially as 'together'/'at the same time').
会議と出張が一緒に重なって困った。
I was in trouble because a meeting and a business trip overlapped.
二つのイベントが一緒に開催された。
Two events were held at the same time.
今日は授業とアルバイトが一緒にあって忙しい。
Today I have classes and a part-time job at the same time, so I'm busy.