A node is a place where two or more elements are connected.
An essential node is a node where three or more elements are connected.
Kirchhoff’s Current Law
The algebraic (signed) sum of all the currents at any node in a circuit equals to zero. Simply put, the charge does not build up at a node.
A closed loop is defined in this way:
Starting at an arbitrarily selected node, trace a closed path in a circuit and return to the original node without passing through any intermediate node more than once. The loops, a, b, c, and d are partial of the closed loops in the circuit.
Therefore, a mesh is a closed loop that does not include any other closed loops. The loops a, b, and c are all meshes.