Marker

In Magic: The Gathering, a marker is an object used to represent anything that isn't represented by a card.

The most common markers are tokens, emblems, and counters. Tokens and emblems are usually represented on the back side of marketing cards found in Magic boosters, while counters are often represented by marbles or dice. Any small object that can be placed on cards or the game surface can serve as a marker, as could notes jotted on paper, even if official markers are available.

Rules

From the Tournament Rules (March 7, 2022—Kamigawa: Neon Dynasty )

3.8 Game Markers
Small items (e.g. glass beads) may be used as markers and placed on top of a player’s library or graveyard as a reminder for in-game effects. These markers may not disguise the number of cards remaining in that zone nor completely obscure any card.

Designations

Some designations are described as "to act as marker".

Monstrous — Monstrosity - Applies to creatures. The +1/+1 counters applied by the ability serve as a physical marker but are not the same thing as the designation itself.
Renowned — Renown - Applies to creatures. Also uses +1/+1 counters in the same way as Monstrous.
City's Blessing — Ascend - Applies to players. A helper card was included in some relevant products (Rivals of Ixalan).

Other designations are not described as a marker by the rules.

Commander — Applies to cards chosen during deck construction. No physical marker, but the designation remains the same throughout the game.
Day/Night — Applies to the game as a whole. A helper card was included in some relevant products (Innistrad: Midnight Hunt and Innistrad: Crimson Vow initially).
Monarch — Applies to players. Marker cards were included in relevant booster packs (Conspiracy: Take the Crown).
Initiative — Applies to players. Marker cards were included in relevant booster packs (Commander Legends: Battle for Baldur's Gate).
Level — Applies to Class enchantment cards. Has no official or implied marker.
Goaded — Applies to creatures.
Protector — Applies to players.
Ring-bearer — Applies to creatures.
Suspected — Applies to creatures. Punch cards with markers were included in relevant booster packs (Murders at Karlov Manor).

Player counters

When counters are placed on a player instead of an object it is often helpful to have a marker card to place these counters on.

Named markers

Turn marker

The Grand Melee variant places great emphasis on turn markers.

From the glossary of the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

Turn Markers: Markers used to keep track of which players are taking turns in a Grand Melee game. See rule 807.4.

Venture marker

Venture markers were introduced in Dungeons & Dragons: Adventures in the Forgotten Realms. If you venture into the dungeon for the first time you put a dungeon of your choice into the command zone and put a venture marker on the first room, at the top. Your venture marker is a way for you to track which room you're in. A venture marker is something small that you can use to keep track of what room you are in, for example, a coin, a bead, a die or a miniature.^[1]

From the glossary of the Comprehensive Rules (June 7, 2024—Modern Horizons 3)