Marginal costing is also known as Variable Costing.It's a 
costing method that includes only variable manufacturing 
costs i.e direct materials, direct labour, and variable 
manufacturing overhead - in the cost of a unit of product.

Marginal costing means variable cost.

marginal cost:it is cost of producing additional unit or cost of producing one more unit

Ascertainment of marginal cost is marginal costing.
Marginal cost :- it is per unit increase or decrease in 
cost of product due to increase or decrease in per unit of 
production.