The sabre saw (also saber saw) is a powered or unpowered hand-held reciprocating saw. The sabre saw consists of a reciprocating cutting implement, usually a toothed blade, to chiefly cut through wood and other various materials. (relative to the jigsaw) its primary use is to slice material handily, rather than using the traditional circular saw in the application of building homes. It's also an advantageous implement when cutting into tight crevices.
The unpowered sabre saw consists of handle that grips an interchangeable blade. It is primarily used in metal working and the building industry in situations where the object to be cut cannot be hindered by the frame of the hand held implement holding the blade, e.g. in metal working the hacksaw's frame doesn't allow for cuts to be made close to the blade where a hindrance would otherwise touch the frame. There are many similarly designed cutting implements that have the traditional sabre saw's qualities but are not known to be sabre saws. These include the keyhole saw, and some Japanese varieties of woodwoorking tools, e.g., the kataba saw.