Investment thesis
An investment thesis is the specific, written reason a position is expected to work out, stated clearly enough that it can later be checked against what happens. It covers what the market seems to be missing or underestimating about the business, and why that difference should close over time.
A good investment thesis also states what would prove it wrong before the position is opened, a lost customer, a shrinking margin, a competitor gaining ground, whatever applies to that specific business. Deciding this in advance matters because it is far easier to judge evidence honestly before money is on the line than after, once the instinct to defend an existing position takes over.
Writing the thesis down, rather than keeping it as a loose feeling, is what turns a pick into something that can be reviewed later. If the original reasoning no longer holds, that's a signal to reconsider the position, regardless of what the price is doing at the time.