gender-neutral honorific or title of courtesy, equivalent to Ma'am, Madam, Miss, Mr, Mrs, Mrs, Sir.
In formal settings, defaulting to gendered titles of courtesy can misgender people. While titles and salutations are not necessarily indicative of gender, there tends to be a corrolation. For example, Ma'am tends to be used to refer to woman- or femme-assumed individuals.
Mx allows you to maintain formality without making assumptions about people's titles/saluations and/or gender.
In the same vein of pronouns, it's important to not make assumptions about which honorific to use for someone and instead ask for explicit consent whenever possible.
Many (though not all) nonbinary people might opt to use Mx.
mix (məks, ˈmiks)