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Let's consider this definition of computational indistinguishability.

Computational indistinguishability. A probability ensemble $X=\{X(a, n)\}_{a \in\{0,1\}^{*} ; n \in \mathbb{N}}$ is an infinite sequence of random variables indexed by $a \in\{0,1\}^{*}$ and $n \in \mathbb{N}$. In the context of secure computation, the value $a$ will represent the parties' inputs and $n$ will represent the security parameter. Two probability ensembles $X=\{X(a, n)\}_{a \in\{0,1\}^{*} ; n \in \mathbb{N}}$ and $Y=\{Y(a, n)\}_{a \in\{0,1\}^{*} ; n \in \mathbb{N}}$ are said to be computationally indistinguishable, denoted by $X \stackrel{c}{\equiv} Y$, if for every non-uniform polynomial-time algorithm $D$ there exists a negligible function $\mu(\cdot)$ such that for every $a \in\{0,1\}^{*}$ and every $n \in \mathbb{N}$, $$ |\operatorname{Pr}[D(X(a, n))=1]-\operatorname{Pr}[D(Y(a, n))=1]| \leq \mu(n) $$

From my understanding $D$ is the distinguishing algorithm, e.g. the adversary in security proofs. An instance of the random variable $X(a,n)$ is considered as the encryption algorithm. However from my understanding only the output of the encryption algorithm, e.g. the ciphertext, is passed to $D$. For people coming from mathematical background this is a bit confusing because random variable is a function $X:Ω \rightarrow Ε$ where $Ω$ is the σ-algebra of the events space and $E$ is a measurable space.

Can someone help me clarify the notation and the definition that is used? Thanks in advance.

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An instance of the random variable 𝑋(𝑎,𝑛) is considered as the encryption algorithm.

I believe an instance of the variable $X(a,n)$ would usually refer to an encryption of a corresponding input $a$. ($X(a,n)$ is the encryption of $a$ with security parameter $n$)

Intuitively, this definition says that if you are given an element from either $X$ or $Y$, it is hard to distinguish which where it came from.

The distinguisher $D$ is given either an element from $X$ or $Y$, and the probability $|\operatorname{Pr}[D(X(a, n))=1]-\operatorname{Pr}[D(Y(a, n))=1]|$ indicates the ability for the distinguisher $D$ to distinguish these. Consider, as an example, if $$|\operatorname{Pr}[D(X(a, n))=1]-\operatorname{Pr}[D(Y(a, n))=1]| = 1,$$ what would this imply about $D$, $X$ and $Y$?

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