my definition for 'conditioning': a preplanned schedule of specific alterations, as compared to standard day-to-day care, that will better prepare that fish for change.
a few examples: 1) fish are often fasted prior to shipping, 2) females are often fed enriched diets prior to being introduced to males, and 3) some species require an alteration to water parameters (temperature, ph, salinity, etc.) to stimulate spawn activities.
i use the term 'conditioning', as a descriptive for maintaining fry in grow out, because there are specific alterations to regular maintenance that will assist with optimum growth potential. special grow formulas, increased water change schedules, and specific filtration choices, to name a few. and as nc_nutcase mentioned: quarantee tanks are often part of the conditioning process for young fish, before they are introduced to potential stress factors, hence the term 'grow out tank'. HTH.