Use this tag for questions that involve any kind of matrix operation where that operation is an inherent part of the question. This includes pattern-matching and extracting (subparts of) matrices, structural manipulation of matrices (reshaping, flattening, rotating) and linear algebra problems like matrix multiplication, transposing, eigenvalue calculation, etc. Generally this tag applies to matrices of any dimension.
Questions specifically dealing with linear algebra should also use linear-algebra. For more general questions dealing with lists (e.g. construction, structural rearrangement, element extraction, etc.) use list-manipulation. For questions about displaying matrices in 2D using typesetting use formatting. For more specific questions related to tensors (e.g. geometric objects that describe linear relations between vectors, scalars, and other tensors) use tensor.