A tensor is a mathematical object. Tensors provide a mathematical framework for solving physics problems in areas such as elasticity, fluid mechanics and general relativity. The word tensor comes from the Latin word tendere meaning "to stretch".
A tensor of order zero (zeroth-order tensor) is a scalar (simple number). A tensor of order one (first-order tensor) is a linear map that maps every vector into a scalar. A vector is a tensor of order one. A tensor of order two (second-order tensor) is a linear map that maps every vector into a vector (e.g. a matrix).