She stared out the window as though she had seen a ghost.

as 在这里是连词，表示"如同"或"好像"的意思。 though 在这里也是连词，表示"虽然"或"尽管"的意思。 as though 是一个固定搭配，表示"好像...似的"或"仿佛"，用来引导一个虚拟的情况或假设。