A lintel is a supporting component above an opening such as a window or door. It carries the relevant construction above towards the sides of the opening. Its material and arrangement need to suit the particular building situation.
How this term relates to your project
A lintel and a window frame do not automatically serve the same structural purpose. Enlarging an opening therefore requires assessment of the support needed and which existing elements can remain. Discuss connections to masonry, insulation and the replacement frame.
External appearance may require a different treatment from the internal finish. A crack above an opening does not independently diagnose a failed lintel; have its cause and condition assessed. When alterations are made, record both the support arrangement and frame perimeter details so structure and finishes are not ordered as unrelated items with conflicting assumptions about the same opening.