Radiant means giving out light, or looking as though you are. A radiant morning, a radiant bride, a face radiant with news she has just been told and cannot keep in.
The happiness sense is the common one — radiant with joy, a radiant smile, a child radiant after a birthday. In physics radiant is technical and neutral: radiant heat travels as rays through the air, which is how a fire warms your face across a cold room. The noun is radiance and the verb behind it is radiate — warmth radiates from a stove, and a person can radiate calm. Radiant is admiring, so keep it for good things; nobody is radiant with disappointment.
1adjectiveradiantShining brightly.
“The lake was radiant in the early sun.”
synonyms: glowing, luminous
2adjectivebeamingShowing great happiness or health in your face.
“She came back from the trip radiant.”
synonyms: beaming
3adjectivesentSent out as rays of heat or light.
“The stove gives off radiant heat rather than warm air.”
Too few real examples of this word to estimate which sense is commonest.
Also accepted in games: beaming
Radiant is a Latin participle in English clothes. It comes from radiāns, the present participle of radiō, "to emit rays or beams", so a radiant thing is a thing caught in the act of throwing out rays. Middle English had it as radyant. The light meaning came first, and the happy face borrowed it.
Latin
radiāns
Middle English
radyant
English, today
radiant
Built from
From Middle English radyant, from Latin radiāns, radiantis, present participle of radiō (“to emit rays or beams”).
Etymology from Wiktionary, CC BY-SA