Input is defined as cryptocurrency being received into a digital wallet.
Cryptocurrency coming into your digital wallet is known as an “input”.
Let’s say you want to spend 10 bitcoin, if you have 3 inputs (3, 9, and 15) then the first two are combined (3+9=12) and both are sent out as outputs. After transaction fees are spent, the remaining balance (about 2 bitcoin) is sent back to you as an input.
Your input is always another’s output. And reversely, your output is always another’s input.