Envelope

The typically white or manilla specially folded paper product (usually sporting an adhesive patch or a latch for sealing) that contains mailed material.

Oh, and there's also a definition related to technical trading.

Technical traders use chart patterns to help them predict future stock movements. Charts visually track the movement of share prices as they bounce around day to day and minute to minute. Those are those jagged lines you see when you look up a stock's price (that's actually just one kind of chart, but it's a good enough visual for what we're explaining here).

There are certain rules of thumb that hold true in most (but certainly not all) cases...technical traders look at a chart, apply these rules of thumb and find places where stocks are likely either to change directions or to keep going in the direction they are going.

Envelopes are a way to apply some of these rules of thumb. An analyst will use certain indicators (moving averages are common) to chart an upper bound and a lower bound. The stock is moving within this envelope. If it gets to the top end, there's a likelihood that it will turn lower. If it gets to the bottom end, there's a likelihood that it will turn higher.

It doesn't always work, but like sports betting, you just have to be right most of the time to make money.