Disclaimer: This list represents what I consider to be the attribute of a strong person. Your opinion could differ and this is a good thing.
what I think of when I see the word strong, photo taken in Tenerife
A creative start of the day
We've started the day at Origo. The subject for today was an article 
I used to consider myself the strongest person and I was seen as such by the ones around me. This meant I was seen as a person not affected by anything around, always calm and smiling, always solving the problems and a shoulder to cry on/ arms to help for the ones in need.
How was this period for me? I was boiling inside as I had so much to show and so much more to be and I refused myself every inch of it. I allowed myself to be only that "strong" person.
Fortunately, I've changed things.
1. A strong person is the person with enough courage to be emotional
Vulnerability has become my mantra. I strongly believe that I am stronger when I let the others see the real me, with all my emotions and face the risk the rejection than when I am just a poker face, always smiling and not giving anything from my true self.
I have some stories here, most of them about friends being afraid of me as they were never sure what are my real thoughts and feelings on the matter. And I was afraid of me also, for the same reasons.
Another issue when I did not let myself to be vulnerable is that I become passive agresive and in an unconscious way hurt the ones that hurt me.
2. A strong person is the person able to say "I am sorry"
Every time I hear people saying, especially in the relation with their children, that I am sorry will affect their authority I try to talk them into another approach.
We all make mistake (if you consider you never did drop me a comment:P ). Through mistakes we learn the most. A mistake is not a premeditated behavior. So why being afraid of saying "I am sorry"?
A fear is usually the one stopping us from asking for forgiveness and a strong person, as I previously said, accepts the emotion and is able to work with it.
So be less afraid and say more: I am sorry.
3. A strong person is the person able to say "I don't know"
This is also about vulnerability and about the acceptance that we cannot know everything. And is ok not to, this is how we feed our curiosity. What would happen if you would know everything? What would be your purpose and the joy in life from that point?
I have a long history with the refusal of asking for help or saying I don't know. It started when I was very little and my brother made fun of me every time I asked for his help. So I've stopped doing this and assumed as personal goal to know everything. Even if I did not knew something I would say I know just to make sure I am not perceived as stupid. I've become a master in inventing things that for the ones not very familiar with the topic would sound true.
Fortunately, things have changed here also and I am amazed daily of how good people react to this very human approach: "I don't know".
4. A strong person is aware of it's past to learn it's lessons
I was 14th when I decided I won't regret anything in my life. And not having regrets at that point was not about knowing that I cannot change things, was about refusing to look back and try to understand what happened.
Took me 10 years to give up this behavior and admit that the past exists and it's my best teacher. If not, it will become my best predictor for future failures.
5. A strong person is aware of the future and the needed present actions to build it
Being aware of the future is not living in the future. The same with the past (knowing where you've came from) is about knowing where are you heading and what you can do today to make it happen.
Staying present is the attitude we all need to live our life at it's full but it worries me how many people use this as an excuse to repeat the same mistakes from the past and to ignore any impact of their actions in the future.
Conclusions
I'm sure everyone describes a powerful person in it's own way. This was my vision,
What is interesting to keep in mind is how much our perception of a strong person correlates with who we already are (if we consider ourselves strong) or who we want to become. So we would be quite subjective on this one. More perspective would only help us get outside our bubble and confort zone and see others as different than us without considering this a bad thing.