Merge down option

Merge down will merge an upper layer with a pixel layer  below and will appear in the menu if a pixel/smart object/text or shape layer above a pixel layer is selected (not an adjustment layer) .

However virtually any two layers (not adjustment layers) can be merged (into a pixel layer if different types e.g. Text and Shape) by highlighting them and selecting Merge Layers (or Merge Shapes if both are shape layers)

Dave

There's flatten image, though it's good practice to keep a layered copy so that you can always return to the original to make further changes if required.