Definition
Bandwidth is the maximum amount of data that a communication line or a path inside a computer can carry in one second. Just as a wider water pipe lets more water pour out at once, a wider data path lets you send more digital information at one time.
Highway Lanes and Water Pipes
Say the speed limit on every road is 60 mph. An 8-lane highway can still move far more cars at the same time than a one-lane road. On a narrow alley, cars have to line up and wait. On a wide highway, lots of cars can roll along together without a jam.
When people say "my internet is fast," they usually don't mean the signal physically travels faster. Most of the time they mean they can send a large amount of data at once. No matter how quickly a signal flies, a narrow path only lets a little through at a time.
How fast an electrical signal moves in a wire, or light moves in a fiber-optic cable, is mostly fixed by the physical properties of the material. (Light in optical fiber moves at about two-thirds of its speed in a vacuum.) So the key to boosting throughput isn't making signals travel faster. It's opening up the path so more information can pour through each second. How quickly a connection responds is a different question, though.
Bandwidth vs. Latency
When people judge the quality of an internet connection, the partner most often mentioned alongside bandwidth is latency. People mix the two up all the time, but they describe completely different things.
The water pipe makes the difference easy to see. Bandwidth is the "width of the pipe," or how much can gush out at once. Latency is the response time of the signal: how long it takes after you turn the faucet before the first drop lands on your hand.
In something like a real-time shooter game, where instant reaction matters, you need low latency so your character moves right away instead of stuttering. On the other hand, when you stream a 4K movie, plenty of bandwidth matters even more than latency. That's what keeps the picture from freezing or breaking up and lets it play smoothly.
A Bit More Precisely
Why does a word about data capacity or path size include "width"? Because the term originally came from radio and wireless communication, where it meant the gap between the highest and lowest frequencies, that is, the width of a frequency range.
To be a bit more precise, it's rooted in information theory: the wider the range of frequencies a signal can use, the more digital data, those 0s and 1s, you can carry in one second. So the name for the width of a frequency band came to be the standard term for the maximum transfer capacity of wired and wireless networks and computer hardware today.
In networks, bandwidth is usually shown in bps, meaning how many bits you send per second. For memory, it's shown in units like GB/s, meaning how many bytes move per second. In wireless communication, it's still used in its original sense of frequency width (Hz). Gigabit internet, 5G mobile networks, and the high bandwidth memory (HBM) used in high-performance AI graphics chips all come from the same effort: dramatically widening bandwidth so huge amounts of data can be processed without a bottleneck.
Common misconceptions
If bandwidth goes up, the physical speed at which data travels gets faster.
The speed of an electrical or light signal moving along a line is nearly constant for the same type of line. Higher bandwidth doesn't mean faster travel. It means the amount of cargo you can carry in the same one second gets bigger.
Where you meet it
When several family members watch Netflix in 4K at the same time, they share the home's bandwidth, so the picture quality may drop for a while.
When you download a huge game on gigabit internet, the wide bandwidth makes it finish much faster. In theory, downloading 50 GB at 1 Gbps takes about 6 minutes and 40 seconds. In practice it takes a bit longer because of overhead.
In one sentence
Bandwidth is the width of a communication path. It's the maximum amount of data that can be carried in one second.