Wraps every head in an expression with Inactive so it is held unevaluated
Inactivate[expr]
Inactivate[expr, patt]
Inactivate[Length[{a, b, c}]]
→ Inactive[Length][{a, b, c}]expr = Inactivate[2 + 2 + 3 ^ 2]
→ Inactive[Plus][2, 2, Inactive[Power][3, 2]]Inactivate[Sum[k ^ 2, {k, 1, n}], Sum]
→ Inactive[Sum][k^2, {k, 1, n}]