From Pluralpedia, the collaborative plurality dictionary
absorbing (n., v., adj.)
Other formsabsorbed (n., adj.)
Applies toheadmates

Absorbing is when a headmate fuses with another headmate while forming.

Usually, they do not realize that they've done so.

It often seems as though the headmate who was absorbed disappeared without a trace. It may be difficult to discern if a headmate was absorbed or not as the headmate who did the absorbing may not show any traits or signs of the absorbed headmate.