A tensor is (put non-rigorously) a generalization of numbers, represented by a multidimensional array with respect to a given basis1.
A tensor is described by its order. An order 0 tensor is a scalar, an order 1 is a vector, an order 2 is a matrix and so on. Tensors are usually denoted using index notation.
1
Tensor. (2021, August 9). In Wikipedia. https://en.wikipedia.org/wiki/Tensor