I noticed some people using the terms as if they were synonoms.
For instance, in the same scenario, I heard "add this function to the lisp image evaluating it" and "eval this function into the Lisp interpreter to use it later".
However, I am not sure the use is technically precise. Thus, the question.
"Image" is a file on disk.
"Add a function to the image"
means evaluate the function and save the image, so the function if immediately available on the next invocation.
"Interpreter" is (usually) a wrong level of abstraction; one should use "REPL" instead. E.g., SBCL does not have an interpreter at all (everything is always compiled) but this is not a detail that is relevant to this topic.
"eval this function into the Lisp interpreter to use it later"
means evaluate the function in the current REPL and use it in the same process (i.e., it is available until Lisp is restarted).
These are two orthogonal concepts. Let’s start from the usually comprehensive Common Lisp Glossary:
Lisp image n. a running instantiation of a Common Lisp implementation. A Lisp image is characterized by a single address space in which any object can directly refer to any another in conformance with this specification, and by a single, common, global environment.
So the key idea is that an image is a set of mutually referring Lisp objects (functions and data) that can be “called” or “accessed” during the execution of a program.
The way in which a Common Lisp program is executed depends instead from the way in which a system is implemented. It could be executed by compilation in machine language, for instance, or through some form of interpretation (or even a mix of the two). So a Lisp interpreter is just a particular way in which an implementation is done (and in the current Common Lisp systems there are many different ways to implement the language).
An image is a copy of a Lisp heap written to disk (or another secondary storage). The Lisp heap is the memory for data storage in RAM of a computer. To write a Lisp heap to an image, the running Lisp is stopped and the memory is dumped to disk. Then the Lisp is either resumed or quit.
The image can be used to restore the heap upon starting a new Lisp. That's usually faster than starting a fresh Lisp and then loading the corresponding software.
A Lisp interpreter is a program which executes Lisp programs from source. Many Lisp implementations don't use an interpreter, but they execute compiled Lisp code, typically Lisp code compiled to native machine code.
