energy level; excitement; spirits; mood
Casual Japanese use, especially in phrases such as テンションが高い, テンションが低い, and テンションが上がる. It often means excitement or mood, not English-like mental strain.
今日はテンションが高いね。
You're really energetic today.
I just can't get excited this morning.
mental strain; anxiety; nervous tension
Rare English-like use for mental tension or anxiety. In ordinary modern Japanese, 緊張 or 不安 is usually more natural for this meaning.
Rarely, テンション can refer to mental strain.
Everyone was in high spirits at the festival.
I'm sleepy in the morning, so I have low energy.
I listened to music before the game to hype myself up.
My mood drops when it rains.