How about: a scene is a single step in the character's journey. They want something (a small step towards their eventual desire), they try to get it, and they either succeed or fail. So the scene has a beginning, middle and end, but it's an integral part of something larger.

A scene is how you bring the reader into your vision for your story. Your desription needs to allow the reader to "see" what you see...be it a description, setting the scene up, or dialogue between your characters of where they are...

Whoa! So simple question - so hard to find the right answer. Maybe it's a piece of a story that follows the rule of three unities? (time, action and place)

Scenes, at least in my view, are the building blocks; they're the bricks that complete the structure, fill in the frame, create stepping stones across the stream, the micro-business of the story, the atoms of the story matter. Scenes are miniature 'stories' in themselves; each needing a beginning point, rising conflict, some point of climax and a resolution that transports the reader to the next building block. As scenes stack one upon the other, the larger conflict unfolds, the characters' dynamics are revealed and the plot becomes evident. At its finest point, a scene is a place where the action occurs. Consequently, scenes require the grand structure of the story to be broken down into its component parts, which might be 'acts' into 'sequences' into 'scenes'. A scene has both a dynamic structure and a purpose, moving the story along in 'beats' that consist of fast/slow pacing and the building and releasing of tension. It's purpose in advancing the story may be manifested through a change in destination, a change in the direction of the action or a change in a character's purpose. It can also be to cause subsequent events, reveal a subplot or new business concerning a character, register or establish a mood or expose one character's actions or involvement in another character's business. Ultimately, the scene needs to show change, decisions, influence, growth or manipulation. In summary, the scene requires a clear understanding of all that has been discussed so far: the structure (plot), complete knowledge of characters, decisions of style, understanding of dialogue dynamics.