A tensor is a generalization of vectors and matrices and is easily understood as a multidimensional array.
A vector is a one-dimensional or first order tensor and a matrix is a two-dimensional or second order tensor. (i.e. N-order tensor is comparable to N-dimensional array)
Tensor notation is much like matrix notation with a capital letter representing a tensor and lowercase letters with subscript integers representing scalar values within the tensor. Below is third order tensor (i.e. 3 dimensional array)
t111, t121, t131 t112, t122, t132 t113, t123, t133
T = (t211, t221, t231), (t212, t222, t232), (t213, t223, t233)
t311, t321, t331 t312, t322, t332 t313, t323, t333
cf. https://machinelearningmastery.com/introduction-to-tensors-for-machine-learning/
