A fit in which the shaft is always larger than the hole, so the parts must be pressed or shrunk together.
An interference fit holds by the friction generated by elastic squeeze, and can transmit substantial torque without a key. The engineering content is in the stress it creates: the hub is in tension and the shaft in compression, and the interference must be large enough to hold the load and small enough not to yield either part.
Practical values start around 0.001 in per inch of diameter for a light press, doubling for a heavy one.
Specifying excessive interference for security, which yields the hub and loses the fit entirely — sometimes with a visible crack.
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