"Cohesive gel" describes silicone gel that is cross-linked so it behaves as a soft solid that holds together, rather than a free-flowing liquid. Almost all modern silicone implants in the UK use cohesive gel.
Why cohesivity matters
Cohesive gel keeps its shape, contributes to a natural feel, and — importantly — limits the spread of gel if the shell is damaged, because the gel tends to stay together. This is a major safety improvement over the soft, liquid-like gels of early implants.
Degrees of cohesivity
Gels range from softer cohesive gels that prioritise a natural feel, to highly cohesive ("gummy bear") gels that are firmer and "form-stable", holding their shape even under pressure. Your surgeon balances feel against shape-control when recommending a gel firmness.