Question

what is bandwidth

Answer 1

Bandwidth is the measure of data speed usually measured in bits per second or factors of it like kilobits or megabits. It essentially is how fast your data travels which translates into what most people consider as "how fast my internet is"

Answer 2

Bandwidth is how wide a section of electromagnetic spectrum is occupied. Wider bands generally allow for more or clearer information transfer, which has lead to the secondary meaning, which is what Spectrum (the user) has described.

Due to this, the exact meaning of bandwidth is going to depend on the application. Let me show two example, both using DSL.

VDSL uses high frequency bands to allow a very wide bandwidth that is able to transmit large volumes of data. However, the high bands get striped by filters and load coils, which makes the wire distances somewhat short. In contrast, IDSL uses lower frequencies that are able to go through filters and coils, allowing greater range, but limiting the bandwidth used and subsequently the data rate is a maximum of 128 kbps.

In that example, the bandwidth is a reference o what range of spectrum is used.

My DSL provider says my maximum bandwidth is 15 megabit, but I usually only get 3.

In this example, the bandwidth is a reference to the speed seen by the end user.

The bandwidth (spectrum) for a particular use is generally fixed and unchanging. However, the bandwidth (speed) that can go over that spectrum is generally variable and conditional on many factors.