What is the meaning of Malleability?

The quality or state of being malleable.

The property by virtue of which a material can be extended in all directions without rupture by the application of load; a material's ability to be bent, formed, or shaped without cracking or breaking.

a property of a cryptographic algorithms in which an adversary can alter a ciphertext such that it decrypts to a related plaintext

Source: wiktionary.org