Opportunity; chance — a single-character noun often used in expressions like 'not miss the chance'.
今が勝負の機だ。
Now is the decisive opportunity.
大きな機会を逃してしまった。
I ended up missing a big opportunity.
機をうかがって行動するべきだ。
You should watch for the right moment and act.
Machine; apparatus — used as a character in compounds for machines or sometimes as an abbreviation for 'machine'.
工場に新しい製造機が導入された。
A new manufacturing machine was introduced to the factory.
古い機がまだ動いている。
An old machine is still running.
織機（おりき）は布を作るための機械だ。
A loom (織機) is a machine for making cloth.
Aircraft — used to refer to aircraft or planes in certain contexts.
敵機が接近していると報告があった。
There was a report that enemy aircraft are approaching.
その空域で複数の機が確認された。
Several aircraft were confirmed in that airspace.
Counter for aircraft (〜機): the numeral classifier used to count planes (e.g., two planes = 二機).
戦闘機が三機離陸した。
Three fighter jets took off.
その空港には何機の旅客機が到着したか分かりません。
I don't know how many passenger planes arrived at that airport.
Used in video games as a counter for remaining lives (as in 残機).
このゲームは残機が5機ある。
This game gives you five extra lives.