Final answer:
The keyword used to define a method in Python is "def". Methods in Python, including the initializer method, are written using this keyword followed by the method's name and any parameters within parentheses.
Step-by-step explanation:
The keyword used to define a method in Python is "def". Methods in Python are defined using the def keyword, followed by the method's name and parentheses enclosing any parameters. The body of the method is indented below the definition line. The initialization method for a class, for instance, is defined as def __init__(self), where self refers to the instance of the class and allows access to the attributes and other methods within the class.