The (socially mandated) dance-unit for dancing with a single specific partner in a formal Argentine Tango dance, also known as a Milonga. Specifically, a group of three or four songs that have some commonality (orchestra, music style, etc) the ending of which is indicated by a snippet of music (known as the "cortina") that is not intended to be danced to (but rebels will). Yes, some DJs play tandas at informal dances (aka "Practica") but that's just plain wrong for various reasons left as an exercise for the reader. To stop dancing with this partner before the tanda is complete is considered rude and unacceptable unless you have a compelling reason. Pleading sudden nasea usually works.
I'm catching my breath this tanda. Want to dance the next one?