The phrase "zoom button" refers to a feature or button on a device or software that allows you to make things on a screen appear bigger or smaller. Full definition
There's a volume rocker / camera zoom button on the left, and power button on the right. (techradar.com)
If you are reading a PDF file, you have to employ the dedicated zoom button. (goodereader.com)
Likewise Huawei offers a 1x / 2x zoom button in these two modes, however it's not an enhanced zoom like the optical zoom on some other dual - camera phones offers, just a quick way to switch between 1x and 2x digital zoom modes, meaning the picture won't be any clearer than if you had just cropped the full resolution image. (androidheadlines.com)