In mechanics, a displacement field is the assignment of displacement vectors for all points in a region or body that are displaced from one state to another. A displacement vector specifies the position of a point or a particle in reference to an origin or to a previous position. For example, a displacement field may be used to describe the effects of deformation on a solid body.

Before considering displacement, the state before deformation must be defined. It is a state in which the coordinates of all points are known and described by the function: $${\displaystyle {\vec {R}}_{0}:\Omega \to P}$$ where

Most often it is a state of the body in which no forces are applied.

Then given any other state of this body in which coordinates of all its points are described as ${\displaystyle {\vec {R}}_{1}}$ the displacement field is the difference between two body states: $${\displaystyle {\vec {u}}={\vec {R}}_{1}-{\vec {R}}_{0}}$$ where ${\displaystyle {\vec {u}}}$ is a displacement field, which for each point of the body specifies a displacement vector.

The displacement of a body has two components: a rigid-body displacement and a deformation.

A change in the configuration of a continuum body can be described by a displacement field. A displacement field is a vector field of all displacement vectors for all particles in the body, which relates the deformed configuration with the undeformed configuration. The distance between any two particles changes if and only if deformation has occurred. If displacement occurs without deformation, then it is a rigid-body displacement.

Two types of displacement gradient tensor may be defined, following the Lagrangian and Eulerian specifications. The displacement of particles indexed by variable i may be expressed as follows. The vector joining the positions of a particle in the undeformed configuration ${\displaystyle P_{i}}$ and deformed configuration ${\displaystyle p_{i}}$ is called the displacement vector, ${\displaystyle p_{i}-P_{i}}$, denoted ${\displaystyle u_{i}}$ or ${\displaystyle U_{i}}$ below.

Using ${\displaystyle \mathbf {X} }$ in place of ${\displaystyle P_{i}}$ and ${\displaystyle \mathbf {x} }$ in place of ${\displaystyle p_{i}\,\!}$, both of which are vectors from the origin of the coordinate system to each respective point, we have the Lagrangian description of the displacement vector: $${\displaystyle \mathbf {u} (\mathbf {X} ,t)=u_{i}\mathbf {e} _{i}}$$ where ${\displaystyle \mathbf {e} _{i}}$ are the unit vectors that define the basis of the material (body-frame) coordinate system.