Timbre is the unique quality that makes one sound different from another.
You might hear someone say "I love the timbre of her voice" - they mean the distinctive way she sounds when she sings. It's not just about how loud or soft she sings, but about what makes her voice stand out. For example, some singers have a warm and rich timbre, while others are brighter and more nasal. When musicians talk about timbre, they're usually thinking about instruments too - like the way a trumpet sounds different from a flute. It's a big part of why we can tell a recording of a piano apart from one made by an electric keyboard.
The quality of a sound independent of its pitch and volume.
