dear alberto,
how do you do ?
the terms "diluted" and "concenterated" are relative terms
they may have exact meaning "concenteration", for example conc. sulphuric-by covention-means 98% concentration and so on.
you -by mentioning the term "saturated" see confused ,as saturation is condion in which the solution cantain maximum amount of solute which may -in some cases - spairingly soluble electrolyte and here it follow the law of mass action
you may consult the following referances:
1.Vogel quantitative analytical chemistry,Longman Publishars.
2-Alexyev quantitative analytical chemistry,Mir Publishars.

Dilute (as a verb) would mean to a chemist that they had to lower the concentration.

Dilute (as an adjective) would mean to a chemist something different for every chemical.

For example: Dilute (v) hydrochloric acid by 10 times = a chemist would have to dilute the HCl so it is ten times less concentrated.

...using dilute (adj) hydrochloric acid.... I would want to be told the concentration (but I guess you wouldn't use conc. HCl (33%?).

The same goes for concentration. Although concentration can be defined as the amount of a compound in a given volume (mol.L-1, g.mL-1 etc) although I guess this could just as easily be referred to as dilution

probably refers to the activity, in pertinence to solvent/analyte relations.  You wouldn't want to use concentration sulfuric acid in titrations, but rather a dilute, perhaps around .1M.

If somebody told you to use dilute acid though there isn't really a standard concentration refered to as dilute, is there? These are surely relative terms.

The concentrated usually meant diminished activity

??

Activity goes down with concentration at first, but later it grows. IIRC 2M CaCl₂ has the same activity coefficients as at 0M concentration, then activity coefficients are getting higher then 1.