One of at least two laser beams used to write holograms. The signal beam is the beam that carries the information to be stored in the holgram. In the case of a holographic picture, this beam is reflected off of the thing you want to take a picture of into the media. In the case of holographic data storage, the beam has some kind information encoded into it (for instance, it can be sent through a transparency or an SLM.)
The other beam necessary to write a hologram is the reference beam.