Is there a word for the stone slab that lays horizontally on top of a grave? In the image it would be on the grave on the left.
There are a couple of options.
Traditionally, the stone covering a grave is called a gravestone or tombstone. Oxford Dictionaries for tombstone:
- A large, flat inscribed stone standing or laid over a grave.
Meanwhile, the Oxford English Dictionary has this to say about gravestone, a term that goes back to Middle English:
- A stone placed over a grave, or at the entrance of a tomb; in later use also applied to an upright stone at the head or foot of a grave, bearing an inscription.
From the 17th century onward the stone at the head of the grave and marking the name and dates of the person buried would be called a headstone. Oxford Dictionaries:
A slab of stone set up at the head of a grave, typically inscribed with the name of the dead person.
However, note that in casual use these three labels aren't always clearly distinguished - the gravestone and tombstone entries have a meaning virtually identical to the headstone. For that reason, if you want to distinguish the covering from the headstone, gravestone slab or tombstone slab is a possible collocation. In Google Books it turns up several technical or archaeological descriptions, like this one from 1900:
Upper right hand corner of a tombstone slab with moulded border, 27 inches by 20 inches, with large letters. Found in the North Wall (West) in November 1890.
That said, the collocation isn't perfect either. I've also found people describing headstones as slabs.