Glossary
Prompt Weighting
Short answer
A method for increasing or decreasing the influence of specific parts of a prompt.
Weighting lets you foreground part of a prompt. In Stable Diffusion it is done with parentheses and a number, in Midjourney with the `::` marker.
Its most common use is forcing a detail the model keeps ignoring: you raise the weight of an element you wrote but that never appears in the image.
Excessive weight degrades the result; the image fixates on that element and the composition falls apart. Start with small increases, around 1.2.
Example
Writing `(red umbrella:1.3)` in Stable Diffusion forces that element to appear in the image.