It seemed like an easy question, but the more I thought about it the more I realized a scene has many qualities. At that moment my answer became more complex.
Let's deconstruct a novel's dramatic structure. Its main component is "story." The next component is "plot." I differentiate between "story" and "plot" because the author might use a different plot to tell the same basic story. A good example would be "Romeo and Juliet," a tragic story of star-crossed lovers that dates back to antiquity. The first published version, The Tragical History of Romeus and Juliet by Arthur Brooke, appeared in verse in 1562 and later in prose as Palace of Pleasure by William Painter in 1567. William Shakespeare's play, which changed and expanded the previous versions, didn't surface until 1597. Romeo and Juliet inspired several later incarnations, including the musical West Side Story, in 1957 and a comic spoof, Romanoff and Juliet, in 1961.
A writer may change the plot and alter scenes that make up the plot, but the basic story remains the same, a tragedy of lovers separated by outside forces.
Once writers have a plot, they choose scenes, a collection of events they believe will best tell the story. I've heard "scene" described as a unit of conflict. That's usually true but it's so much more. It's a component of time, place and most importantly change. It should move the story forward, support the story's theme and premise (terms I explained in my July 2013 blog), reveal character, spark emotion in the reader (suspense, thrills, chills, etc.), bring the story to life through vivid sensory details (sounds, touch, smells), answer questions from previous scenes and introduce new story questions that will keep the reader reading.
While story, plot and scene must have conflict that rises to a climax (that point when the protagonist wins or loses), it also must have scenes of dénouement or resolution (that moment when the story's questions are answered and when the protagonist may—or may not— enjoy the rewards of her struggle).
Choose your scenes wisely. Ask yourself what purpose does the scene serve. It should serve at least two purposes, if not more. For example, the protagonist might conquer an obstacle that moves her closer to the goal but then she encounters a more difficult, more dangerous obstruction. At the same time, the scene should reveal the protagonist's character through her struggles. The harder she struggles, the more she is likely to win the reader's sympathy. Ask yourself how many purposes each of your scenes serves—and how you can add more.
I would be happy to hear your thoughts on the question, "What is a scene?"