an error, a mistake. a lapsus calami is an error while writing, while a lapsus linguae would be a slip of the tongue.
The word is the past participle of the deponent Latin verb "labor, laberis, lapsus sum, labi" that means "to slide, to fall, to glide down, to sink".
The word has also another, rather technical, meaning, in ecclesiastical Latin: a lapsus, in the Early Church context, was a Christian that had fallen back into paganism.