Muria was a fish sauce like garum. It was less expensive than garum as it was made from tuna instead of mackerel.
Because it was not as dear, it is more likely to have been used instead of garum by common folk or soldiers in their supplies.
In some places in the Empire and at various times throughout the Empire’s history, the word Muria appears to have been used as well to mean the brine after fish were pickled in it.
Literature & Lore
“The ancients extracted from fish two highly flavoured seasonings, muria and garum. The first was the juice of the thuny, or to speak more precisely, the liquid substance which salt causes to flow from the fish.” — Jean Anthelme Brillat-Savarin, The Physiology of Taste, Dec 1825.