It will depend on the context.
I recall the term "clamping" being used. It appears in various disciplines including numerical optimization and computer graphics.
In the computer graphics area this is needed to distinguish it from an important, ubiquitous, but different operation called "clipping"
Despite that, "clipping" is used in signal processing to denote your operation.
There is a closely allied operation in statistics called "Winsorizing". Winsorizing can be construed as beginning with a data-dependent clipping operation.
This graph of the "clipping" or "clamping" operation was created by plotting the function $x \to a \vee (b\wedge x)$ (where $\vee$ is the maximum and $\wedge$ is the minimum). It visually demonstrates that
ClippingClamping is piecewise linear.
It can be construed as a special form of linear "spline" connecting the points $(a,a)$ and $(b,b).$ (Applying affine transformations to either or both coordinate will transform it into a linear spline between any pair of distinct points.) See https://stats.stackexchange.com/a/291598/919 for the theory and code.