It has different implications in different contexts. In general, delta defines the number of units of an asset that should be purchased to hedge one unit of a liability. As far as derivatives concerned, delta (as a greek) measures the sensitivity of of a derivative price to the movement in the underlying asset/ price/ rate, etc.

For example, a 50% delta is supposed to imply that the derivative is just half as sensitive as its underlying asset, and that the position holder will need two dollars in face value of the derivative to replicate a one-dollar movement in the asset price. In options, it is the units of an underlying asset (e.g. the number of shares) to buy for each option sold in order for the investor to create a safe investment position.

Delta could also be computed for a forward contract (delta for a forward), for a forward-forward (delta for a forward-forward), a futures contract (delta for a futures), a bond (delta for a bond), etc.