Like a row in a table. I/O instructions in S/360 (as I recall) assembler would fetch an entire record in a single instruction / cycle (the I/O devices are radically different and would support this - it wasn't just syntactic sugar around reading a word or byte at a time). So the time to read in a record, perform some operations and write a record back out is actually much more predictable based on clock speed than it sounds like it would be to someone from a PC background.