Double-entry bookkeeping
every operation changes at least two lines; Δassets = Δthe right side of the balance sheet (liabilities + equity)
In more detail
A rule of accounting: one operation always changes at least two lines, and after it the columns add up again. A loan is made — both sides grow. The customer repays — both shrink.
This is not an accounting formality but a way of not lying to yourself: with one entry you can draw any wealth you like. The second entry always asks — at whose expense.
In the academy “Double-entry bookkeeping” is not a definition but an operation: you post it yourself and watch what it does to the balance sheet.