Rigorous is nearly akin to rigid, but is a stronger word, having reference to action or active qualities, as rigid does to state or character; a rigid rule may be rigorously enforced.

J. C. Fernald, English Synonyms and Antonyms (1914)

Example sentences

thorough We will check everything in the most thorough way.
stringent We need to introduce more stringent security measures.
meticulous Sami Bakir has been a meticulous investigator.
scrupulous Tom is scrupulous in matters of business.
strict Algeria has to relax its strict economic policies.

Popularity over time

Word	1850 → 2019	Trend
rigorous		rising
thorough		falling
stringent		steady
meticulous		steady
scrupulous		steady
strict		steady

Use in printed books, per decade from 1850 to 2019.

By length

4 letters: hard¹
5 letters: tough¹, tight¹, exact², harsh¹, stern¹, rigid²
6 letters: severe², strict¹, brutal²
7 letters: precise², austere²
8 letters: accurate³, thorough²
9 letters: demanding³, stringent², draconian⁴
10 letters: meticulous⁴, inflexible⁴, unyielding³, scrupulous³
11 letters: painstaking³, unrelenting⁴, punctilious⁴

The small number after each word is its number of syllables (exact² = 2 syllables).

In other languages

Français: rigoureux
Italiano: rigoroso
Español: riguroso
Català: rigorós