Hybrid is a term used to describe a financial instrument which displays characteristics of both debt and equity.
Such instruments might be designed to be an intermediate (or mezzanine) category of capital between equity and debt, or to have some of the risk absorbing characteristics of equity and, ideally, the tax efficiency of debt.
These are 'hybrid' financial instruments.
The term 'hybrid' can also refer to an entity which is treated differently for tax purposes in different tax jurisdictions.
More broadly, any structure, instrument or entity with mixed, or intermediate, characteristics between two or more other, simpler or standardised structures.