I've never thought about the definition of gameplay, always just taking it as a general term for how fun a game is to play. I suppose, primarily though, I think of it as how the player moves around, getting from point A to point B, how much the player is involved with that process and in what ways.
Gameplay, beyond the general explanation, is more than that though. From what I've found, the term can often be used to describe the levels of playing within the game and how they affect one another.
Normally these levels will be a local scale and a global scale as used in strategy games. Recently, Lionhead Studios tried to incorporate these effects more dominantly in 'Fable 2'. While it seems like a good concept that doing evil deeds will make you look more evil and affect how the people around feel about you and vice versa, this must also lead to consideration of other aspects of game design. I found that because of this freedom to change your in-game personality at will, that I could not form a connection with the character. One minute I'm being called a hero, the next, a complete !*£$%", and how am I supposed to get attached to my family when they're completely materialistic and use me for my gifts? The dog's the best part. Anyway, that's more relevant to my previous blog but I just had to air my thoughts.
Back to the issue of gameplay. As I said earlier it can also be used loosely as a way to describe how to get from A to B. Games such as 'Portal' and 'Braid' could be said to have a more unique gameplay than others by using theories of time travel and other dimensions so to speak, which also means using your brain! Shock, horror, I know. I've never concentrated on a game so much as Portal in quite a while.
Then there's also the physical movement of your character and what he/she or it can do. Games can thrive on a new way to play just because it's different to what we're accustomed to and people catch on. 'Crackdown' and 'Mirrors Edge' for example have taken gameplay to new heights (excuse the pun, couldn't resist) by allowing the player a greater freedom of movement over large areas & heights. By doing this, they had to develop a new way to move around and interact with the environment, and this encourages the player to take a more mentally active involvement in the game in my opinion.