1
a
: the reddish brittle coating formed on iron especially when chemically attacked by moist air and composed essentially of hydrated ferric oxide
b
: a comparable coating produced on a metal other than iron by corrosion
c
: something resembling rust : accretion
2
: corrosive or injurious influence or effect
3
: any of numerous destructive diseases of plants produced by fungi (order Uredinales) and characterized by usually reddish-brown pustular lesions
also
: a fungus causing this
4
: a strong reddish brown
rusted; rusting; rusts
1
: to form rust : become oxidized
iron rusts
2
: to degenerate especially from inaction, lack of use, or passage of time
most men would … have allowed their faculties to rust—
T. B. Macaulay
T. B. Macaulay3
: to become reddish brown as if with rust
the leaves slowly rusted
4
: to be affected with a rust fungus
1
: to cause (a metal) to form rust
keep up your bright swords, for the dew will rust them—William Shakespeare
William Shakespeare2
: to impair or corrode by or as if by time, inactivity, or deleterious use
3
: to cause to become reddish brown : turn the color of rust
Examples of rust in a Sentence
Noun
an old pump covered in rust
Verb
Your bicycle will rust if you leave it out in the rain.
The moist air rusted the latch on the door.
The old iron fence was rusted.
Recent Examples on the Web
Katelyn Squiers, Better Homes & Gardens, 1 Feb. 2025
Salt can cause rust, which can damage equipment involving braking, steering, suspension and fuel systems, according to Consumer Reports.
—Evan Moore, Charlotte Observer, 31 Jan. 2025
Ashlyn Needham, Southern Living, 30 Jan. 2025
Meanwhile, a few thousand of the old vehicles rusted away in storage.
—David Axe, Forbes, 19 Jan. 2025
See all Example Sentences for rust 
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Noun
Our answer: When aluminum foil comes into contact with rust, a chemical reaction occurs that reverses the oxidation process that created the rust in the first place.
—Katelyn Squiers, Better Homes & Gardens, 1 Feb. 2025
Salt can cause rust, which can damage equipment involving braking, steering, suspension and fuel systems, according to Consumer Reports.
—Evan Moore, Charlotte Observer, 31 Jan. 2025
Verb
Not to mention, tools that have rusted can become dangerous and need to be tossed immediately for safety precautions.
—Ashlyn Needham, Southern Living, 30 Jan. 2025
Meanwhile, a few thousand of the old vehicles rusted away in storage.
—David Axe, Forbes, 19 Jan. 2025
See all Example Sentences for rust
Dictionary Entries Near rust
Cite this Entry
“Rust.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/rust. Accessed 11 Feb. 2025.
Copy CitationKids Definition
rust
1 of 2 noun1
: a reddish brittle coating formed on iron especially when chemically attacked by moist air and composed chiefly of ferric oxide
2
a
: any of numerous destructive diseases of plants caused by fungi and characterized by reddish brown spots
b
: any of an order of fungi that cause plant rusts
3
: a reddish brown
rust
2 of 2 verb
: to form or cause to form rust
iron rusts
Medical Definition
rust
noun1
: the reddish brittle coating formed on iron especially when chemically attacked by moist air and composed essentially of hydrated ferric oxide
2
: any of numerous destructive diseases of plants produced by fungi (order Uredinales) and characterized by reddish brown pustular lesions
also
: a fungus causing this
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