MatrixSymbol

MatrixSymbol[a] represents a matrix with name a.

MatrixSymbol[a,{m,n}] represents an mn matrix.

MatrixSymbol[a,{m,n},dom] represents a matrix with elements in the domain dom.

MatrixSymbol[a,{m,n},dom, sym] represents a matrix with the symmetry sym.

Please visit the official Wolfram Language Reference for more details and examples on core symbols.