Icon System
An icon system is a coordinated set of pictograms with consistent stroke weight, grid size, corner treatment and level of detail. Icons support quick orientation in navigation, forms or service overviews, and only work when they look consistent and are unambiguously understood.
In practice
Do not mix icon sets: as soon as a filled symbol sits next to a line symbol, the page looks careless even when nobody can say why. Judge every icon by comprehensibility rather than originality, since a cog for settings is boring and understood, whereas an inventive symbol for "consulting" is not. Icons do not replace labels: a symbol without text is a guessing game for many users and worthless to screen readers without an accessible name. Technically, icons belong on a website as SVG so they stay sharp and can take on brand colours. Decide who is allowed to add new icons, or the set will grow uncontrolled.