I doubt that there's any formal definition.
Water company emergency teams will be on call 24 hours per day. My guess is that whoever booked your home visit into their computer system could see that there would be nobody free before, say, 6pm and so simply told you that you'd receive a visit 'this evening'. Even they probably had no real idea of whether that meant 7pm or 11pm.
If a job that higher up on the list than yours gets finished much faster than expected, you might get a visit in the near future. However if a job that was expected to take just an hour or so ends up taking four or five hours, you might not see anyone until nearly (or even after) midnight.
Sorry!