Lacan borrows the term "code" from Roman Jakobson's theory of communication.
Jakobson presents his opposition "code vs message" as an equivalent of Saussure's langue vs parole.
Code and Language
However, Lacan draws an important distinction between the concepts of language and code.
Codes are the province of animal communication, not of intersubjective communication.
Index and Signifier
Whereas the elements of a language are signifiers, the elements of a code are indices.
The fundamental difference is that there is a fixed bi-univocal (one-to-one) relationship between an index and its referent, whereas there is no such relationship between a signifier and a referent or between a signifier and a signified.
Ambiguity and Equivocation
Because of the bi-univocal relation of indices and referents, codes lack what Lacan regards as the fundamental feature of human languages: the potential for ambiguity and equivocation.
Lacan is not always consistent in maintaining this opposition between code and language.
In the seminar of 1958-9, for example, when presenting the elementary cell of the graph of desire, he designates one point as the code, which he also designates as the place of the Other and the battery of signifiers.
In this case, it is clear that the term "code" is being used in the same sense as the term "language," namely, to designate the set of signifiers available to the subject.