Definition
A spark plug whose firing end has become coated or bridged with a deposit -- typically lead, carbon, or oil -- that prevents it from producing a strong spark across the electrode gap. The deposit conducts electricity to ground or insulates the electrodes, so the high-voltage pulse from the magneto leaks away or fails to jump the gap, and the cylinder misfires.
Plain English
The tip of the spark plug has gotten dirty enough that it can't make a proper spark anymore, so that cylinder doesn't fire correctly.
Context Anchor
Pilots most often encounter this during engine start, run-up, or any time the engine runs rough or shows reduced power.
Derivation
Foul comes from Old English ful, meaning dirty or impure. The word kept that meaning in mechanical use: a fouled part is one that has been clogged or contaminated by buildup.
Why Pilots Care
A fouled plug causes misfiring, power loss, or hard starting and can lead to an in-flight engine issue if not addressed.
Analogy
It is like trying to light a match with the striking surface covered in grease. The match may be fine, but the dirt keeps the spark from doing its job.
Intuition Check
Do not assume “fouled” means the whole spark plug is physically damaged. Here it usually means deposits or contamination are interfering with the spark.
Example Sentence 1
During run-up the right magneto dropped 250 RPM, so the instructor suspected a fouled spark plug and had the student lean the mixture and run the engine up to clear it.
Example Sentence 2
After an extended idle with a rich mixture the mechanic removed and cleaned the fouled spark plugs before the next flight.