Question

The title is the question: Are the words “lexer” and “parser” synonyms, or are they different? It seems that Wikipedia uses the words interchangeably, but English is not my native language so I can’t be sure.

Answer 1

No. Lexer breaks up input stream into “words”; parser discovers syntactic structure between such “words”. For instance, given input:

velocity = path / time;

lexer output is:

velocity (identifier)
= (assignment operator)
path (identifier)
/ (binary operator)
time (identifier)
; (statement separator)

and then the parser can establish the following structure:

= (assign)
  lvalue: velocity
  rvalue: result of
    / (division)
      dividend: contents of variable "path"
      divisor: contents of variable "time"