def METHOD_NAME(gb_file):
    """
    Parse the *DEFINITION* field of a GenBank or GenPept file.
    Parameters
    ----------
    gb_file : GenBankFile
        The GenBank file to read the *DEFINITION* field from.
    Returns
    -------
    definition : str
        Content of the *DEFINITION* field.
    Examples
    --------
    >>> import os.path
    >>> file = GenBankFile.read(os.path.join(path_to_sequences, "ec_bl21.gb"))
    >>> print(get_definition(file))
    Escherichia coli BL21(DE3), complete genome.
    """
    lines, _ = _expect_single_field(gb_file, "DEFINITION")
    return " ".join([line.strip() for line in lines])