radial

Jacob Christian Hansen Ellehammer invented the first 3 cylinder radial internal combustion engine in 1904.

The "normal" engine is also called a rotary engine. Both of these engines look very similar. Radial engines use a conventional crankshaft in a fixed engine block, unlike the rotary engine.

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You've missed the fundamental point. A conventional engine has its cylinders in a straight line or lines. A radial engine's cylinders are arranged, as its name says, radially from the polygonal crankcase, like the spokes of a wheel.

Pratt & Whitney nine cylinder radial engine

The fact that both are aircraft engines is the only connection.