A short pointed double-bladed knife, commonly carried in period.
During the middle ages this weapon was primarily used in what some now refer as a reverse grip. ie hilt up and point down. During the renaissance and later, the dagger came to be held point up in the manner of a knife except that the blade would be twisted in the hand so that the thumb rested against the flat of the blade.
Hilts of earlier daggers were simple and might be of the bollock type (having a guard that resemble testicles), rondel type or others. Renaissance daggers might have curved quillions, that might catch other blades and could also have a basket hilt. This basket hilt was placed slightly different to basket hilts on swords as the guard would often be symetrical and on the face of one side of the sword rather than the edge.
Blades varied (between times and cultures) to encompass from the stiletto -- thin and needle-shaped -- to the cinqueada, almost five-fingers broad (hence the name) at the hilt, which reduces down to a point quite quickly. [Interestingly (or not) both of these originated from Italy.]
It was thought of by some as being a "poetic" weapon of choice for assassins.