Account portability is a broad label for what Money Wiki covers in detail as bank account portability: the ability of a customer to take an account relationship from one provider to another with as little disruption as possible. This page explains what people usually mean when they use the shorter phrase.
In its loosest sense, account portability is a policy goal rather than a single service. It covers being able to close an old account easily, have salary payments, direct debits and standing orders moved to the new account, and, in the strongest version, keep the same account number when changing bank, much as a mobile phone number can be kept when changing network. Full number portability of that kind is rare in banking, because account numbers are tied to a bank's sort code or routing details.
Regulators usually deliver a weaker but practical form of it. In the European Union the Payment Accounts Directive requires banks to run a switching service, and the United Kingdom has its own redirection service. Those mechanisms are described under payment account switching. The term "account portability" also appears in open banking debates, where data portability lets a customer carry transaction history to a new provider.