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The term elastic thickness refers to the thickness of an elastic layer of material, which along with an indication of its elasticity (e.g., Young's modulus) determines its elastic behavior when sagging under some weight. The term is used to refer to the lithosphere, the flexible upper layer of the Earth's surface, which includes the crust and a bit more, which shows some elasticity given the (very slow) sagging under loads (e.g., volcanos, glaciers), lithospheric flexure. The lithosphere is relatively hard and resistant to flexure over the course of a year or a human lifetime, but over geologic time, it is sufficiently flexible that it will sag into the mantel underneath like a fabric cover of a swimming pool with a weighty object on a portion of it.
The term elastic thickness is commonly used as short for effective elastic thickness (aka apparent elastic thickness or equivalent elastic thickness, Te), the thickness the material would have to be to produce the observed sag, presuming some value of Young's modulus. Using a good estimate of the average Young's modulus value, the effective elastic thickness is something of a model-approximation of the actual lithosphere's thickness at various locations. The Young's modulus value used in such models may be an average of the entire Earth, or may be a reasonable/plausible or known value for the region. The term elastic thickness is generally used for derivations based upon gravimetry. Seismic data also offers information about the thickness of the Earth's layers, but unfortunately the two sources of information are sensitive to below-surface characteristics that are not fully related.
The term normal elastic thickness (Tn) indicates an elastic thickness presumed or determined to be the usual thickness for a region.