Solid Oak is 100% natural oak throughout the entire piece. Engineered Oak has a thin oak veneer on top of a core, typically plywood, MDF, or HDF.
Solid Oak has a natural grain visible on all surfaces and can be sanded and refinished multiple times. Engineered or veneered oak only has a slim top layer of oak therefore its abilty to be sanded or refinished is limited (usually 1–2 times maximum).