Digital pianos with line out

Linear

— an output that allows you to broadcast the analogue audio signal from the digital piano to external equipment such as powered speakers, power amplifier, computer, etc. Note that, unlike MIDI (see the relevant paragraph), this output receives an already “ready” sound, which only needs to be amplified and sent to the speakers. The line interface can be useful, for example, for connecting additional speakers and providing the required sound power, for connecting to a sound recording console, etc. The connector itself most often refers to the mini-Jack 3.5 mm or Jack 6.35 mm standard — these are the most popular interfaces in modern audio technology, moreover, they are mutually compatible through the simplest adapters.

Also note that some instruments have more than one line input — this allows you to output sound to several external devices at once.