The way I always considered tanking was whether it would get you on the hot seat to be fired if you were a "real-life" GM.
(1) Lose 110 games or more. I think, in any case, if you lose 110 games or more, that's tanking as a season like that in real life would almost certainly cause a rash of firings.
(2) If you have a crap team with a bloated payroll and some young talent that isn't ready (still in 1st or 2nd MiL seasons), I think it's fine to trade some of the ****** contracts, limp by with some weaker stop-gap FAs, and win 60-65 games again. I see that as a situation where, in real life, fans would understand for a year, maybe two. Even if you traded a good player, as long as your win total didn't decrease, and you got a good prospect, I'm fine with that. There are some teams that have such a screwed-over financial situation, that spending a year to untangle the mess is acceptable, as long as you at least spend a little on some stop-gap FAs and keep the team able to win 60+.
(3) If you have a crap team that's not totally hamstrung with a crap payroll and an utter lack of talent, you should be improving your win total... or at least making moves that have the clear intent to improve the win total. This includes promoting star prospects who are in their 3rd or 4th MiL seasons, if need be. Imagine the Nationals trying to leave Strasburg in the minors until Bryce Harper and whoever they pick next year were ready to come up as well. That would be an outrage. However, this doesn't mean that you need to shell out big bucks to 33 year-old FAs. In real life, you wouldn't be asked to go from 60 wins to 90. But you would be expected to go from 60-70. If you inherit a 60-win team (that's not in a total financial debacle), you should be either promoting a few prospects to improve your team, or (if you want to leave them down because they're not ready), you should at least pick up one or two decent FAs. They don't need to be top-end, but spend 5M a year to pick up a solid 3rd/4th-starter that will keep you from having to trot out a waiver-wire guy until your stud prospect is ready.
(4) If your team won 70+ the previous year, there's absolutely no reason you shouldn't be trying to improve it, and if you don't, that's tanking, to me. You can win 70 games and still have tanked, IMO, if you weren't making any moves to try to win. Besides, you never know, I won my division in Happy Jack last season with 76 wins, so going from 72 wins to 78 wins (not worth it according to some on here) can mean a playoff spot every now and then.