The figure called the Merkaba in modern sacred geometry is a compound of two regular tetrahedra sharing a common centre, one inverted with respect to the other. It is a real and well-studied solid. Its eight outer points are the vertices of a cube; the region where the two tetrahedra intersect is a regular octahedron; and the whole can equally be described as an octahedron with a pyramid raised on each of its eight faces. Seen along the right axis its silhouette is the six-pointed star, which is why it is sometimes described as a three-dimensional Star of David.
Its documented history is Renaissance rather than esoteric. It was illustrated by Leonardo da Vinci for Luca Pacioli's De Divina Proportione, printed in Venice in 1509, where it appears as an elevated octahedron. Johannes Kepler named it the stella octangula in the early seventeenth century, treating it as a compound of two tetrahedra. At no point in that history did it carry a mystical meaning.
The identification of this solid with the word Merkabah is recent. It was set out and popularised by Drunvalo Melchizedek from the mid-1980s: the teaching that a field of two counter-rotating tetrahedra of light surrounds the human body, that it can be activated through specific breathing exercises, and that it functions as a vehicle, drawing on the chariot sense of the word, for travel between dimensions or states. The spelling Merkaba, without the final consonant, is often used to mark this modern sense, though the two spellings are not applied consistently.
Read as a modern spiritual practice with a forty-year history, it is what it is, and it has a substantial following. Read as a recovery of what Ezekiel saw or what the Hekhalot mystics practised, it is not supportable. No text in that tradition mentions tetrahedra, counter-rotation, or a geometric body of light, and the connection between the two senses of the word is the word itself and nothing more.