The US Department of Defense use still a different, more specific definition. More complete definitions exist, but they're harder to find.
"LANGUAGE PROFICIENCY LEVEL DEFINITION
Language Proficiency Level I:
Speaking:
Reading:
Able to satisfy minimum courtesy requirements and maintain very simple face-to-
face conversations on familiar topics.
Sufficient comprehension to read very simple connected written material.
Language Proficiency Level II:
Speaking:
Reading:
Able to satisfy most work requirements with language usage that is acceptable
and effective. Able to speak the language with sufficient structural
accuracy and vocabulary to participate effectively in most formal and
informal conversations on practical, social and professional topics.
Able to read within a normal range of speed and with almost complete comprehension a
variety of material on unfamiliar subjects. Can comprehend a variety of styles and forms
pertinent to professional needs. Rarely misinterprets texts.
Language Proficiency Level III:
Speaking:
Reading:
Able to use the language fluently and accurately on all levels normally pertinent to
professional needs. Language usage and ability to function are fully successful.
Able to read fluently and accurately all styles and forms of the language pertinent to
professional needs."