Diminish — ruling 1

Diminish overwrites all previous effects that set the target creature’s power and toughness to specific values. Other effects that set its power or toughness to specific values that start to apply after Diminish resolves will overwrite this effect.

Diminish — ruling 2

Effects that modify the target creature’s power or toughness, such as the effects of Elesh Norn, Grand Cenobite or prowess, will apply to it no matter when they started to take effect. The same is true for counters that change the creature’s power or toughness (such as +1/+1 counters).

Diminish : règles, fonctionnement et interactions (MTG)

Diminish is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Target creature has base power and toughness 1/1 until end of turn.

How it works : Diminish

Original English text — Wizards of the Coast / Scryfall

#1November 17, 2017· Wizards of the Coast
Diminish overwrites all previous effects that set the target creature’s power and toughness to specific values. Other effects that set its power or toughness to specific values that start to apply after Diminish resolves will overwrite this effect.
No other card is named in this ruling.
#2November 17, 2017· Wizards of the Coast
Effects that modify the target creature’s power or toughness, such as the effects of Elesh Norn, Grand Cenobite or prowess, will apply to it no matter when they started to take effect. The same is true for counters that change the creature’s power or toughness (such as +1/+1 counters).
No other card is named in this ruling.

Interactions with other cards

These cards' text mentions Diminish.

Diminished Returner

Diminished Returner enters tapped. {B}{B}: Diminished Returner perpetually gets -1/-1, then return it to the battlefield. Activate only if Diminished Returner is in your graveyard and its toughness is 2 or greater.