Intensive means packed into a short time or a small space, with everything turned up. A two-week intensive Spanish course, six hours a day, no weekend; a patient in intensive care with a nurse reading the monitors all night.
The word attaches to effort and resources: intensive training, intensive farming, an intensive search of the woods before dark. Economics pairs it with what a job swallows, so labour-intensive work needs many pairs of hands and capital-intensive work needs expensive machines. Keep it apart from intense, which is the mistake almost every learner makes: intense is a strong feeling or degree, as in intense heat or an intense stare, while intensive is concentrated effort. Heat can be intense; a course cannot. The adverb is intensively, and the noun intensity belongs to intense rather than to this word.
1adjectiveintensiveConcentrating a great deal of work, attention or care into a short period.
“She signed up for an intensive driving course two weeks before the test.”
synonyms: concentrated, rigorous
2adjectiveusingUsing a large amount of one particular resource for each unit of output.
“Rice growing here is labour-intensive, with every field planted by hand.”
Intensive traces back to Latin intensus, meaning attentive or eager, the participle of intendo, to stretch out or strain. That stretching passed through Medieval Latin intensivus and Old French intensif into Middle English intensive, where it meant fervent. Behind the whole line sits the ancient root tend-, to stretch, which is exactly what an intensive week does to you.
Proto-Indo-European
*tend-
Latin
intēnsus — attentive; eager, intent; intensive
Medieval Latin
intēnsīvus
Old French
intensif
Middle English
intensive — fervent, great, intense
English, today
intensive
The adjective is derived from Late Middle English intensive (“fervent, great, intense”), borrowed from Old French intensif, intensive (modern French intensif) + Middle English -ive, equivalent to intense + -ive. Intensif is from Medieval Latin intēnsīvus, from Latin intēnsus (“attentive; eager, intent; intensive”) + -īvus; and intēnsus is the perfect passive participle of intendō (“to stretch out, strain”), from in- + tendō (“to extend, stretch, stretch out”) (ultimately from Proto-Indo-European tend- (“to extend, stretch”)). a doublet of intend. The noun is derived from the adjective.
Etymology from Wiktionary, CC BY-SA