Definition
The 48 adjoining states of the United States plus the District of Columbia, all sharing common borders within a single continuous land area. Alaska and Hawaii are excluded because they do not share borders with the other states.
Plain English
The 48 states in the lower part of the U.S. that all touch each other, plus Washington, D.C. Alaska and Hawaii are not included.
Context Anchor
Seen in FAA weather-service descriptions, including discussions of where services such as HIWAS are available.
Derivation
From Latin con- (together) and terminus (boundary or border). Literally 'sharing a boundary.' That is exactly what the term means here: the states that share borders with each other in one continuous block.
Why Pilots Care
Many FAA weather advisories and products apply only to the conterminous states, requiring separate sources or procedures when flying in Alaska or Hawaii.
Intuition Check
Do not read conterminous states as all U.S. states. Here it means the connected lower 48 states only.
Example Sentence 1
HIWAS broadcasts hazardous weather information over selected VOR stations throughout the conterminous states.
Example Sentence 2
A pilot planning a flight in the conterminous states checked the latest hazardous weather information before departure.