netsu

Netsu is a Japanese noun associated with heat or fever, derived from the kanji 熱. In everyday language it covers physical warmth, the condition of having a fever, and, more broadly, a sense of intensity or ardor. The fever sense is commonly spoken as ねつ (netsu), as in 発熱 (hatsunetsu, fever) or ねつがある (netsu ga aru, to have a fever). Beyond medical usage, netsu appears in phrases that express passion or enthusiasm.

In compounds, netsu functions as an on’yomi element, appearing in scientific terms as well as figurative expressions. Examples include 熱量 (netsuryō, amount of heat) and 熱力学 (netsuryokugaku, thermodynamics). Other related words use the same kanji with different readings, such as 情熱 (jōnetsu, passion), 熱心 (nesshin, enthusiasm or zeal), and 熱中 (netsuchū, absorption or immersion in something). These terms illustrate how 熱 can convey both physical heat and metaphorical intensity.

Etymology and readings: 熱 has the on’yomi netsu and the kun’yomi atsui, the latter appearing in adjectives meaning “hot.” In compound form, netsu often contributes the sense of heat or fervor. The word netsu thus covers a broad semantic field, from measurable thermal energy to the human drive or zeal that fuels actions and interests.

See also: 熱量, 熱力学, 情熱, 熱心, 熱中.