Final answer:
Cn in the expression t = | C1 of t1 | ... | Cn of tn represents one of several constructors in an algebraic data type of a functional programming language, where tn indicates the type of data associated with the constructor Cn.
Step-by-step explanation:
In the context of programming, specifically in algebraic data types commonly found in functional programming languages, Cn represents one possible constructor of a union type t. The expression t = | C1 of t1 | ... | Cn of tn defines a type t that can be any one of several different constructors (C1, C2, ..., Cn), each potentially carrying its type of associated data (t1, t2, ..., tn). Each constructor Cn acts as a unique tag that identifies the type of data the instance of the type t contains at runtime, and tn represents the type of data that the constructor Cn can hold.
For example, in a programming language like OCaml or Haskell, you might define a type for a shape that can be either a circle with a radius or a rectangle with a width and height. This would look something like type shape = | Circle of float | Rectangle of float * float. Here, Circle and Rectangle are the constructors, akin to Cn in your expression, with float and float * float being their associated data types.