The terms "A1 Starter," "A2 Elementary," "B1 Pre-Intermediate," "B2 Intermediate-Plus," "C1 Advanced," and "C2 Unabridged" are used to describe different proficiency levels in the English language. These levels are part of the Common European Framework of Reference for Languages (CEFR), which is a globally recognized standard for measuring language ability. The CEFR is divided into three broad categories: A (basic user), B (independent user), and C (proficient user).

Here's a brief description of each level:

• A1 Starter: This is the beginning level of English language proficiency, where the focus is on basic survival skills such as introducing oneself, asking for directions, ordering food, etc.

• A2 Elementary: At this level, students can understand and use simple English to describe their experiences, interests, and daily routines.

• B1 Pre-Intermediate: At this level, students are able to understand and use more complex language, and engage in more advanced conversations and negotiations.

• B2 Intermediate-Plus: This level represents a higher level of proficiency, where students are able to communicate effectively in most everyday situations and have a good understanding of idiomatic expressions and more complex grammar structures.

• C1 Advanced: This level is characterized by the ability to communicate effectively and fluently in English, and to use the language accurately and appropriately in a wide range of situations.

• C2 Unabridged: This is the highest level of proficiency, where students have near-native fluency in English and can use the language accurately, appropriately, and effectively in any situation.

These proficiency levels are useful for both students and teachers, as they provide a clear and standardized way of measuring and describing language ability, and help to identify areas for improvement.