Plain English Meaning
If you read a gauge from an angle instead of looking at it head-on, you can read the wrong number. That mistake is parallax error.
Full Definition
An error in reading an instrument caused by viewing its pointer or scale from an angle rather than straight on. Because the pointer sits slightly above the dial face, looking at it from the side makes it appear to line up with a different number than it actually indicates.
Context Anchor
Seen when reading analog cockpit instruments, fuel gauges, sight gauges, compass cards, or any marked scale where your eye position can affect what the reading appears to be.
Derivation
From the Greek 'parallaxis,' meaning 'alteration' or 'change.' The same word describes how a nearby object appears to shift against a distant background when you move your head side to side. In a cockpit, that small shift between the needle and the dial behind it is exactly what causes the misreading.
Analogy
If you look at a ruler from the side, a pencil tip may seem to line up with one mark even though it lines up with another when viewed straight on. The object did not move; your viewing angle changed the apparent reading.
Intuition Check
Parallax error is not caused by the instrument changing its value.
It is caused by your eye being off to the side when you read it.
Example Sentence 1
The instructor reminded the student that reading the altimeter from the right seat could introduce parallax error, so she leaned over to check it directly.
Example Sentence 2
Reading the magnetic compass from the side introduced parallax error and caused a heading misinterpretation during the turn.