Surface/cortical tension	Surface tension: Tension resulting from forces acting in the plane of the surface of a liquid tending to minimize its surface area. In the context of a rounded cell, both cortical tension and membrane tension contribute to the cell’s effective surface tension, with cortical tension dominating (Tinevez et al., 2009; Fischer-Friedrich et al., 2014). Contractile elements of the cortical acto-myosin cytoskeleton are cause of the tension generation in the cortex.
Intracellular pressure	In a fluid-filled spherical object like a cell (or balloon) at steady-state, the pressure difference between the external and internal pressure is related to surface tension and curvature according to Laplace law (see below). Tension-build up in the cortex results in an increase in (hydrostatic) pressure. In the cell, intracellular pressure changes can also arise from water flux across the membrane (due to ion fluxes across the membrane).
Cortical stiffness	Resistance of the cell cortex to bending under force. This depends on its viscoelastic properties, cortical tension, geometry and experimental settings. Also referred to as cortex rigidity.
Laplace law	Describes the relationship between surface tension T, the pressure difference between the inside and outside ΔP and the radius of curvature R (e.g., for a sphere) in a fluid-filled spherical object (such as a rounded cell) (Yeung and Evans, 1989).
Stress	Defined as force (F) per unit area (A). The type of stress depends on how forces act, e.g., normal stress (σ = F/A) (perpendicular on surface, either compressive or tensile) or shear stress (τ = F/A) (parallel to surface).
Strain	Ratio of deformation in direction of the force (ΔL) relative to initial length (L), e.g., normal strain ε = ΔL/L or shear strain γ = δ/L (both dimensionless).
Elasticity	Ability of a material to resist a deforming force and to return to its initial shape upon force removal.
Elastic modulus	Ratio of stress and strain, quantifies the resistance of an object to elastic deformation upon stress application. There are three moduli defined depending on force direction application: the Youngs modulus E, the shear modulus G, and the compression/bulk modulus K (see below). These moduli are related for a linear-elastic isotropic material as: E = 2G(1+ν) = 3K(1-2ν) = 9KG/(3K+G) (ν: Poisson ratio); two of them are sufficient to capture the elastic behavior of a material (Landau and Lifshitz, 1986).
Young’s modulus E	Relates normal stress σ and normal strain ε (or how much stress is applied to obtain a certain level of strain), e.g., for simple case of uniaxial deformation σ = Eε (Hook’s law for one-dimensional case).
Shear modulus G	Relates shear stress τ and shear strain γ, τ = Gγ.
Bulk/compression modulus	Describes how much change of pressure is needed for a certain volume change (volumetric elasticity).
Viscoelasticity	Property of a material displaying both elastic and viscous mechanical behavior. Viscoelastic materials display a frequency dependent stress-strain response. Therefore, the timescale at which they are probed matters.
Complex, storage & loss modulus	The complex moduli G∗ or E∗ can be defined for viscoelastic materials (analogous to the elastic moduli, Shear and Young’s moduli, for elastic materials). G* = G′+iG” (or E∗ = E′+iE″, respectively) with its real part, the storage modulus G′ (E′) and its imaginary part, the loss modulus G”(E”). The storage modulus relates to the ability of a material to store energy elastically, while the loss modulus is related to the ability of a material to dissipate energy. Can be measured in oscillatory measurements, as in phase (storage) and out of phase components (loss) of the stress to strain response (Jacobs et al., 2012).