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thermal

THUR-muhl /ˈθɜːrml/

nounadjectiveB1

neutral

Underwear designed to retain body heat, typically made of a waffle-weave fabric or high-tech synthetic fibers.

While 'thermal' can refer broadly to anything heat-related, in clothing it specifically denotes the insulating base layer worn in cold weather, often called 'long johns.'

The story behind the word. The root therme has given us words like thermometer and thermostat. In the early 20th century, manufacturers began applying 'thermal' to garments knitted with a waffle pattern, which traps pockets of warm air next to the skin, creating an insulating barrier.

Word relationships

long johns: Long johns is a casual, traditional term for two-piece thermal underwear, whereas thermal is the broader, modern technical term.
base layer: Base layer is an athletic term that includes moisture-wicking summer gear, whereas thermal specifically implies heat retention.

Commonly confused with

fleece: Fleece is a thick, fuzzy mid-layer fabric, whereas thermals are thin, close-fitting base layers worn directly against the skin.

Word family

thermally (adverb): In a way that relates to heat or temperature.
thermodynamics (noun): The branch of physics dealing with heat and other forms of energy.

Collocations

thermal underwear
thermal top
thermal leggings
waffle-knit thermal

Example sentences

"Make sure to pack high-quality thermals to wear under your ski pants, as temperatures on the mountain drop rapidly." A packing list for a ski trip.
"The construction workers wore heavy-duty thermal underwear to endure the freezing winter winds." A description of outdoor work gear.

Memory hook

Think of a 'thermometer' which measures heat. 'Thermal' underwear is what keeps your personal heat high. Thermal: Traps your body heat like a personal thermos.

When not to use

Do not use to describe standard, lightweight summer underwear or decorative lingerie.

Fun facts

The classic waffle-knit pattern of thermals was inspired by the military, which needed a lightweight, breathable, yet highly insulating fabric for soldiers serving in freezing climates.