# Definition:Indiscernible

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## Definition

Let $\MM$ be an $\LL$-structure.

Let $I$ be an infinite set.

Let $X = \set {x_i \in \MM: i \in I}$ be an infinite subset of the universe of $\MM$ indexed by $I$.

For:

- every $n \in \N$

and:

- every pair of subsets $\set {i_1, \ldots, i_n}$ and $\set {j_1, \ldots, j_n}$ of $I$ each with $n$ distinct elements,

Let:

- $\MM \models \map \phi {x_{i_1}, \ldots, x_{i_n} } \leftrightarrow \map \phi {x_{j_1}, \ldots, x_{j_n} }$ for all $\LL$-formulas $\phi$ with $n$ free variables.

Then $X$ is **(an) indiscernible (set) in $\MM$**.

Informally, $X$ is **indiscernible (set)** if and only if $\MM$ cannot distinguish between same-sized ordered tuples over $X$ using $\LL$-formulas.

## Also known as

Elements of an **indiscernible set** are often called **indiscernibles**.

## Also see

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