Gender transition, expression and identity tool kit, published by CBRE, suggests non-binary is: ‘…an umbrella term for gender identities that fall outside the gender binary of male or female. This includes individuals whose gender identity is neither exclusively male nor female, a combination of male and female, or between or beyond genders. Similar to the usage of transgender, people under the non-binary umbrella may describe themselves using one or more of a wide variety of terms.'
Stonewall suggests non-binary as: ‘An umbrella term for people whose gender identity doesn’t sit comfortably with ‘man’ or ‘woman’. Non-binary identities are varied and can include people who identify with some aspects of binary identities, while others reject them entirely.’ Ref https://www.stonewall.org.uk/help-advice/faqs-and-glossary/glossary-terms