A dead short is a connection with zero/near-zero resistance.
It may be applied deliberately in order to perform tests, or, more usually, as the result of a catastrophic failure in some part/component.
"The positive and negative voltage lines are tied together - OR" - both lines of an a.c. supply are connected together.
"the positive line is shunted to ground with voltage applied" - OR - the negative line is shunted to ground, etc etc.