What this suffix does
The suffix -osis names two types of thing:
1. A disease or chronic/degenerative pathological state: fibrosis (build-up of fibrous tissue), cirrhosis (hardening of the liver), tuberculosis, psychosis (severe mental disorder), neurosis.
2. A biological process or transformation: metamorphosis, symbiosis (mutually beneficial relationship), diagnosis (process of identifying a disease), osmosis (passage of liquid through a membrane).
Spanish: fibrosis, psicosis, neurosis, metamorfosis, simbiosis — almost always identical.
Key difference from -itis
Two medical suffixes that are easy to confuse:
-itis = inflammation (acute, usually with clear immediate symptoms): appendicitis, meningitis, bronchitis.
-osis = chronic or degenerative process/condition (not necessarily inflammation): fibrosis (tissue build-up), cirrhosis (hardening), arthrosis (degenerative joint wear without active inflammation).
Example: arthritis (inflammation of a joint) vs arthrosis (degenerative wear of a joint without active inflammation). Modern medical English tends to use "osteoarthritis" for both.
Diagnosis and osmosis: the two most everyday uses
"Diagnosis" is probably the most widely used -osis word outside specialist medicine:
"What's the diagnosis?" → common in everyday speech.
Plural: diagnoses /daɪəɡˈnəʊsiːz/.
"Osmosis" has a literal meaning (biology: water passing through a semi-permeable membrane) and a very common figurative use:
"I picked it up by osmosis" = I absorbed it gradually without conscious effort.
This figurative use is extremely frequent in everyday English.