Why are greases called thixotropic gels?

Thyrotrophic gels are those or fluids that are viscous under normal condition become thin or less viscous over time when agitated (shaken) or otherwise stressed. Similarly, greases on agitation or on being stressed become fluid or less viscous which regain their viscosity on standing or removal of stress. Hence, greases are known as thixotropic gels.

Thixotropic gels, also known as greases, are 1-part materials that flow when pressure is applied and remain in place when the force is removed (like Vaseline). These materials are generally used to displace the air gap between two optical elements.