Straight-line wind is any thunderstorm wind that is not associated with rotation. The term exists mainly to separate this wind, and the damage it does, from tornadic wind.
Where the wind comes from
Most damaging thunderstorm wind at the ground starts as a downdraft. Rain-cooled air sinks out of the storm, reaches the ground and spreads outward as outflow.
A downburst is the general name for a strong downdraft that produces an outward burst of damaging wind, and a microburst is a small, concentrated one. The leading edge of the outflow is the gust front, where rain-cooled air meets the warmer air flowing into the storm.
In bowing lines, a rear-inflow jet, a band of strong wind that starts in the trailing rain area and points toward the leading edge, can strengthen the surface wind. The National Weather Service (NWS) office in Louisville notes that bow echo winds and downbursts account for a large majority of structural damage from non-tornadic thunderstorm wind.
How strong it gets
The NWS counts a thunderstorm as severe when it produces winds of at least 58 mph (50 knots), hail at least 1 inch in diameter, or a tornado. The National Severe Storms Laboratory (NSSL) notes that these winds can reach 100 mph.
NSSL also notes that severe thunderstorm wind damage makes up about half of all severe reports in the lower 48 states and is more common than tornado damage.
When a fast-moving band of storms produces a long swath of wind damage, the event may be classified as a derecho. One common guide, from NSSL, is a swath of more than 240 miles (about 400 km) with gusts of at least 58 mph along most of its length.
Reading the damage
Damage surveys help separate straight-line wind from a tornado. The NWS office in Binghamton notes that microburst damage often looks flattened, with larger uprooted trees pointing the same way or fanning out in a divergent pattern, while tornado damage tends to look chaotic, with trees crossing in a convergent pattern.
At the scale of one building, wind is wind. The NWS office in Wakefield notes that trees and structures hit by severe straight-line wind or by a weak tornado experience similar damage.
What the label does not tell you
Straight-line describes the lack of rotation, not the strength. The Wakefield office notes that squall lines typically produce straight-line winds of 60 to 80 mph and can span 50 to 100 miles, a far wider footprint than a weak tornado of similar intensity.
The wind is also not ruler-straight. Outflow spreads, curves and gusts around terrain and buildings, so the term means no organized rotation rather than a perfectly straight path.