The hearth is found at the very bottom of a fireplace. It’s the slab of non-combustible material that sits flat at the base of the fireplace on which a fire or fireplace unit sits.
It can be found surrounded by the walls of the fireplace inside the firebox, and extends out into the room past the fireplace opening.
Below is a diagram I put together showing which part of the fireplace is the hearth:
A hearth is sometimes referred to as the ‘inner hearth’ and ‘outer hearth’, located inside and outside of the firebox respectively. In some cases the hearth is the floor located outside of the firebox, while the inner floor of the fireplace can sometimes be known as the ‘firebox floor’. The extension of the hearth into the room from the fireplace opening is also referred to as the ‘hearth extension’.
For the purposes of this article the hearth will refer to the entire floor area of a fireplace.