1. Archaic term for a register. On-line use of it as a synonym for register is a fairly reliable indication that the user has been around for quite a while and/or that the architecture under discussion is quite old. The term in full is almost never used of microprocessor registers, for example, though symbolic names for arithmetic registers beginning in ‘A’ derive from historical use of the term accumulator (and not, actually, from ‘arithmetic’). Confusingly, though, an ‘A’ register name prefix may also stand for address, as for example on the Motorola 680x0 family.
2. A register being used for arithmetic or logic (as opposed to addressing or a loop index), especially one being used to accumulate a sum or count of many items. This use is in context of a particular routine or stretch of code. “The FOOBAZ routine uses A3 as an accumulator.”
3. One's in-basket (esp. among old-timers who might use sense 1). “You want this reviewed? Sure, just put it in the accumulator.” (See stack.)