Intense means extremely strong in degree — of heat, feeling or effort. Intense heat, intense pain, intense concentration.
Of a person it means serious and emotionally concentrated, which can be admiring or wary: he is quite intense. Note the pair intense and intensive: intense describes strength, and intensive describes concentrated effort over a short time — an intensive course, intensive care. An intense course would be a strange thing to say. Note also intensify and intensity, and that intents in to all intents and purposes is a different word.
1adjectiveintenseExtremely strong in degree.
“The heat in the desert was so intense that we stayed in the shade.”
2adjectiveseriousSerious and emotionally concentrated, of a person.
“He is intense company and worth the effort.”
Too few real examples of this word to estimate which sense is commonest.
intense is a degree of strength.
Latin intensus means stretched tight, from tendere, to stretch — the same verb behind tense, tendon and attention. Something intense is under strain, which is exactly how intensity feels.
Proto-Indo-European
*tend- — to extend, stretch
Latin
intēnsus — strained, stretched tight; intense; attentive; violent; (rare) eager, intent
Old French
intense
Middle English
intens
English, today
intense
Built from
From Late Middle English intens, intense (“ardent, fervent; extreme, great, intense”), borrowed from Old French intense (modern French intense), or directly from its etymon Latin intēnsus (“strained, stretched tight; intense; attentive; violent; (rare) eager, intent”), the perfect passive participle of intendō (“to stretch out, strain”), from in- + tendō (“to extend, stretch”) (ultimately from Proto-Indo-European tend- (“to extend, stretch”)).
Etymology from Wiktionary, CC BY-SA