Plain English Meaning
The official little signs and colored marks inside the airplane that tell the pilot the rules and limits for flying that specific airplane. They are not random stickers — they are part of what makes the airplane legal to fly.
Full Definition
The labels, signs, and instrument markings that the FAA has officially accepted as part of an airplane's certification, displayed in the cockpit or on instruments to inform the pilot of operating limitations, procedures, or restrictions specific to that aircraft.
Context Anchor
You see approved placards/markings in the cockpit, on instruments, near controls, and sometimes on the outside of the airplane during preflight or normal operation.