Uniform
A value handed to a shader that stays the same for every pixel in a frame, like time.
See it in Shader forge. Press + Add uniform, then drag its slider and watch the shader respond.
In plain words
A shader runs once for every pixel, and each run knows only its own pixel. To share something with all of them, the page sends a uniform. Time is the classic one: every pixel reads the same seconds, so the whole picture moves together. The mouse position and the screen size are often uniforms too. They can change between frames, but never within one.