Ligature
Two or more characters drawn as one merged glyph, often to stop shapes like f and i from crashing, sometimes just for style.
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What it is
A ligature is one glyph that stands in for two or more characters. Dodging a collision is the most common reason to draw one: in 'fi' the hood of the f crashes into the dot of the i, so the font draws a single shape where they share a stroke. Plenty of others exist purely for style, history, or the writing system, with nothing crashing at all. You still typed two letters, and copy, search, and screen readers still see two letters. Only the drawing changed.
Three flavors worth knowing. Standard ligatures ('liga': fi, fl, ffi) are on by default and you should leave them alone in body text. Discretionary ligatures ('dlig': st, ct, sp with a connecting swash) are decorative, off by default, and belong in wedding invitations more than in a dashboard. Coding ligatures are the programmer variant: Fira Code and JetBrains Mono fuse '=>' into an arrow and '!==' into one compact symbol.
Gotcha: ligatures fight letter-spacing. Once you track out a headline, the fused pair looks like a mistake, so kill them with 'font-variant-ligatures: none'. Separately, do not paste the precomposed Unicode characters (U+FB01 and friends) into your copy: those really are one character, and they break find-on-page and spellcheck.
Ask AI for it
Keep standard ligatures on for body copy but turn them off for the tracked-out display headings: set 'font-variant-ligatures: none' on headings with letter-spacing above 0.05em, and leave the default 'liga' behavior everywhere else. For the code blocks, switch to JetBrains Mono with coding ligatures enabled so '=>', '!==' and '->' render as fused symbols. Do not use precomposed Unicode ligature characters in the text itself.