Interesting question. I think about it a lot so i'll give you an idea what goes on in my head
To me, "strong" is simply overcoming.
For one person it may be a 400lbs DL. For another it might be re gaining the ability to walk, or just getting kicked in the face by life but getting up, dusting yourself off, and going right back after it.
I think in our world strength should just be defined as making progress and getting sh*t done. If I squat 500 by the end of the year i'll be considered strong by some. If I blow out a knee tomorrow and can squat an empty bar in three months I'll be considered strong by others.
I don't think it's something that can be defined physically. It's a mental thing. Whatever "strong" is to you, it's always just your mind taking your body for a ride.
In other words, I think "strong" runs deeper than a bar and some plates.
If you're being training/gym specific then it probably depends on the enviroment. In my gym a 225lbs bench is considered impressive. Any squat over 300lbs is impressive and and DL in and around the 400lbs mark is impressive. You're considered VERY strong in my gym if you can do any of these. To me personally, I consider a 300lbs bench, 400 squat, and 500 deadlift to be "decent"/respectable (this is for me personally).