Volume.
The displacement method refers to immersing in a liquid. If the liquid is in a container with volume measurement marks, then the object's volume is the difference in the liquid's volume, measured before and after immersion.
There's a legend that the discovery of the displacement method was made by Archimedes when entering a bathtub full of water, which overflowed. This is postulated as the source of the exclamation "Eureka!" ("I have found it").