What does it mean "to play (music, piano) expressively" ?

[quasimodo]

Seriously, it's a very vague concept to me. How would you define it ?

[le_poete_mourant]

I think it has to do with the tone with which you strike the key, so that when the hammer hits the string it's hits you right there too.  It's the difference between a flat, harsh sound, and a pure, singing sound.  It's when you can take what you are experiencing emotionally and plug it into the music much like you would a mathematical equation, making the music personal. 

It's vague, I think, because people are expressive in different ways.  Interpretation is boundless. 

[dnephi]

Phrasing, balance, accents, rubato, and having loose wrists.

Feel the music through your soul, and let yourself express it.

[myotherself]

Its all in the release baby.

[quasimodo]

Its all in the release baby.

Care to elaborate?

[counterpoint]

Its all in the release baby.

I don't know if this is true. The word "expressive" means to press something out, this will not work in a state of total relaxation. I wouldn't say Glenn Gould played very relaxed, but he played very expressive 

[nortti]

I think it has to do with the tone with which you strike the key, so that when the hammer hits the string it's hits you right there too.  It's the difference between a flat, harsh sound, and a pure, singing sound.

I've always wondered whether this is important or not. To me it seems it's not really possibly to play a single note on a piano in many different ways (beyond volume, length, how you release and stuff you can do with the pedals). The most important thing seems to be how the note sounds when compared to previous notes and other notes in the current chord/other voices.

[arbisley]

oh i do believe that you can sound harsh and tinny on the one hand, and soft, gentle melodious and expressive on the other.
or just simply boring...