What is the name of the lintel, or load bearing beam, that forms the base of the entablature in a Greek temple?
The name of the lintel, or load-bearing beam, that forms the base of the entablature in a Greek temple is called the architrave. The Greek root arch- means "first" or "beginning" and the "trav-" root comes from a Latin word (trabs) that means "beam" or "timber". Thus, the architrave is the foundational or main beam.
When looking at a temple, one can see the architrave running horizontally above the very top part of a column (the abacus) and just below the frieze, which in a temple of the Doric order is comprised of triglyphs and metopes.