Definition
The 48 adjoining states of the United States plus the District of Columbia, sharing common borders with one another. Excludes Alaska, Hawaii, and US territories.
Plain English
The 48 lower states that all touch each other, plus Washington, D.C. Alaska and Hawaii are not included because they don't share a border with the others.
Context Anchor
You may see this term in FAA guidance, weather information, airspace descriptions, or service-area limits that apply only to the connected mainland United States.
Derivation
From Latin 'con-' (together) and 'terminus' (boundary). Conterminous literally means 'sharing a boundary.' That is exactly what the 48 lower states do — each one touches at least one other.
Why Pilots Care
Many FAA weather services, flight planning tools, and regulations apply only to this area, affecting route planning and briefing requirements for flights that remain inside it.
Intuition Check
Do not read Conterminous US as “all of the United States.” In FAA use, it means the connected lower 48 states plus the District of Columbia, excluding Alaska, Hawaii, and territories.
Example Sentence 1
The new procedure applies throughout the conterminous US but not in Alaska or Hawaii.
Example Sentence 2
Cross-country flight planning for the Conterminous US uses standard sectional charts without the special procedures required for Alaska.