Squeeze is the ratio of the amount of deformation applied to the seal expressed as a percentage of the free-state cross-sectional thickness.  Deforming the seal cross-section "energizes" the elastomer matrix, causing the rubber material to push back against the mating components.

The greater the squeeze, the more force is applied and the tighter the seal.  But that doesn't mean the most squeeze is the best.

Beyond a certain level, other factors intervene that can work against an effective seal.

With higher squeeze comes more friction and faster wear in dynamic applications.  It also provides a higher risk that pinching will occur when the o-ring is installed.

The force that the squeezed elastomer exerts against the mating hardware tends to decay with time.  The o-ring will retain its squeezed shape even when it is no longer squeezed.  This is called "compression set."  When compression set reaches 80%, most o-rings are in danger of losing their ability to seal.

Material also plays a large part in squeeze.  Nitrile is more tolerant of squeeze compared to perfluorinated elastomers which may rupture when squeezed more than 30%.

Consequences of a seal that is too tight:

1.  Possible damage of pinching during installation

2.  More friction and wear in dynamic applications.

3.  Greater and faster "compression set."

Proper installation provides you with the best seal, with the longest operational life and lower maintenance costs and downtime.