Unification is a fundamental process in Prolog that allows the interpreter to match and combine terms, variables, and predicates. It forms the basis for pattern matching, logical inference, and the execution of Prolog programs. Unification is guided by the goal of making two terms identical by finding substitutions for variables.

In Prolog, unification works as follows :

1. Matching Constants : If the terms to be unified are constants (atoms, numbers, or other non-variable terms), they must be identical for unification to succeed. If the constants match, unification succeeds, and no variable bindings occur.

2. Matching Variables : If one or both of the terms to be unified are variables, unification always succeeds, and the variable(s) are bound to the other term(s). If both terms are variables, they are bound to each other, establishing an equivalence.

3. Matching Complex Terms : If the terms to be unified are complex (compound terms or predicates), the unification process is recursive. Prolog attempts to unify corresponding subterms based on their positions within the terms. The following conditions must be met for successful unification of complex terms:
   * The functor (predicate name) must be the same.
   * The arity (number of arguments) must be the same.
   * Corresponding subterms are unified recursively. During unification, logical variables act as placeholders that can be bound to values. When a variable is encountered, Prolog checks its current binding. If it is unbound, it becomes bound to the corresponding term. If it is already bound, the unification process attempts to unify the values.

Unification is used in various Prolog operations, such as querying, pattern matching, rule application, and logical inference. It allows Prolog to establish relationships, find solutions, and explore the search space. The backtracking mechanism in Prolog utilizes unification to undo bindings and explore alternative possibilities.

Prolog uses a form of unification known as Robinson's unification algorithm, which handles variable bindings, occurs-check (to prevent infinite terms), and supports complex term unification efficiently.