Paper stock / substrate
The actual material you print on: its weight, coating, color, and texture. Same file, different stock, completely different object.
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What it is
Stock is the material you print on, described by a bundle of properties: weight, coating, color, surface texture, opacity, and brightness. 'Substrate' is the wider word that also covers vinyl, fabric, corrugated board, styrene, and anything else a press can feed. Two prints from identical files on different stock read as two different products, which is why stock choice is a design decision, not a procurement afterthought.
Pick it early, because it drives everything downstream. Uncoated stock soaks up ink and dulls saturated colors; coated stock holds detail and punchy color but shows fingerprints in gloss. Textured and colored stocks eat fine hairlines and small reversed type. Recycled and unbleached sheets shift your whites warm, so a 'white' logo prints as the paper, not as ink.
Gotcha: never approve stock from a screen or a photo of a sample. Get physical swatches from the actual printer, hold them under the light the piece will live in, and if budget allows, get a printed sample on the exact sheet. Mills also discontinue lines, so lock availability before you spec a house stock in brand guidelines.
Ask AI for it
Recommend a paper stock for this piece and justify it. Specify: sheet family and finish (coated gloss, coated matte, or uncoated), grammage in gsm, shade (bright white, natural, or cream), and any texture such as laid, felt, or linen. Explain how the choice affects ink saturation, dot gain, fine type legibility, and the tactile feel in hand, then name two commonly available alternatives at different price points.