Optical alignment
Nudging things off their measured position so they look aligned, because the eye judges shapes, not bounding boxes.
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What it is
Your eye aligns shapes, not bounding boxes. A play triangle centered by the numbers looks shoved left, because its visual mass sits toward the flat edge; nudge it a couple of pixels right and it finally looks centered. Same story with round letters overshooting the baseline and cap height, quote marks pushing a paragraph's first line out of the column, and an icon plus label pair that measures perfect and reads crooked. Optical alignment is moving things off their measured position so they look correct.
Type designers live here: overshoot on O and S, sidebearings tuned per glyph, hanging punctuation (also called optical margin alignment, which InDesign ships as a toggle and CSS approximates with hanging-punctuation). In UI it shows up as icon nudges, logo lockups, big display headings pulled left so the letterform edge sits flush with the text below, and buttons whose vertical padding is uneven so the label sits optically centered.
Gotcha: the fix scales with size. A one-pixel nudge that saves a 48px icon is invisible at 16px and wrong at 200px, so prefer relative units and only do this where it is actually visible. And document the nudge. An undocumented magic offset in a component reads like a bug to the next person and gets 'fixed' back.
Ask AI for it
Apply optical alignment corrections to this UI. Shift the play triangle inside the circular button about 8% of its width to the right so it looks centered rather than measuring centered. Pull large display headings left by their left sidebearing so the letterform edge sits flush with the body text below, and hang opening quote marks and bullets into the margin. Rebalance button padding so the label looks vertically centered (usually 1 to 2px less padding below than above, since text sits high in its line box). Use em-relative offsets so the corrections scale with type size, and add a comment on each one saying it is an optical nudge.