stupid question im sure, but......

im sure youll get a bunch of different definitions on here for me...when Im lifting in the 4-6 rep range I consider that heavy

It depends on the person - it means different things based on your level of strength. Lifting heavy should be a weight that causes you to fail in 6-8 reps, while maintaining correct form.

thank you for proving my point haha nice

I would deem it anything that lights up your central nervous system (CNS). Like if you're lifting to work up to, and improve upon your 1 rep max. The heaviest you can lift, is obviously a single rep. So you may do a lifting session where you do 1-1-1-1-1-1 as-in a single heavy rep (like 85%), then rest, now do another single that increases to 90% and so on until the last set you've doing a 1rm or failing.
Other scenarios might be where you do 5 sets of 5 reps where you weight is like 80% of 1rm. You should be going to almost failing at the beginning, and potentially failing at the end. The 5x5 has an end goal to actually test and improve your 1 rep max ... as opposed to "just getting a sweat on".
I honestly wouldn't classify bicep curls, or tricep presses, or small isolation exercises as lifting heavy either - those are assistance exercises - certainly, the weight may increase but there isn't a threshold here - doing "1 rep" of a bicep curl is heavy, but probably worthless and just asking for an injury.
Lifting heavy is big lifts that recruit your posterior chain and CNS to do - like using lifts like Deadlift, Squats Overhead Press, Cleans, Jerks, Snatches. Something that recruits a lot of the body and really taxes the central nervous system.
Curls and such are assistance exercises that do not do this, sure, they may be "heavy" but they are really only assistance exercises. You get them stronger so you can recruit them to lift something really heavy .... preferably from the ground to overhead, or where you need your entire posterior chain to do move that weight.

At the end of the day, you'll know when you are truly lifting heavy. Your entire body will tell you - your CNS will be on fire. If it's only a bicep or a calf crying out, then that isn't what I'd consider "lifting heavy" - that just means you used a heavy weight to do an isolation exercise. You really didn't lift anything.
You'll also notice the difference between doing a heavy leg press on a machine, versus a heavy front squat with a barbell. The press will certainly wax your thighs, but the front squat will wax you thighs and core. Both are heavy .... but one is only a real lift that taxes the entire body.