On a 94 Nissan pathfinder just above the starter there is a wire leading to the engine block what is it?

if it is a big power wire ( same size as the battery cable ) then it is the earth wire you should find that the battery has one wire going directly on to the engine or chassis

there are different arangements the negative wire could be coming directly to the starter and if this is the case then the wire is just earthing the engine but in a lot of cases the starter is getting its ground from the engine and the engine gets its ground from the battery