Plain English Meaning
A striped or crossed-line warning pattern that pops up on an instrument's face to tell the pilot the instrument isn't working properly and its reading should not be trusted.
Full Definition
A warning indicator displayed on certain cockpit instruments — typically the airspeed indicator or Mach/airspeed indicator — consisting of a pattern of crossed diagonal lines that appears in the instrument's display window when the instrument's information is unreliable, unpowered, or has failed.
Context Anchor
Seen on some altimeter faces and in pitot/static instrument diagrams, especially when learning how to read altitude indications.
Derivation
The word 'crosshatch' comes from the drawing of shading an with two sets of crossing each other. The uses the same pattern, which is why it carries the name.