An ELT Glossary : The Plateau Stage

The plateau stage (or phase) often occurs at intermediate level and refers to a period during which the learner "stops learning".  Yi (2007) has described it as follows : 

... as the learning process goes on, the learner finds it harder and harder to take in new language data. The teacher also finds that his input, no matter how much he or she tries to make it interesting, is no longer as easily taken in by the learners as it used to be. The students are more and more discouraged by the fact that their ambition of mastering English as a means of communication turns out to be a false assumption. They find that they know a lot about the English language, but they can hardly say they know English. 

It is during this period of time that many EFL learners suffer great anxiety and eventually give up their efforts to learn the English language. 

In my personal experience I've found it often happens at around CEFR B1 level , though Richards (2008) places it higher, at B2. He describes five characteristics of the plateau level :

1. There is a gap between receptive and productive competence.  
2.  Fluency may have progressed at the expense of complexity.  
3.  Learners have a limited vocabulary range. 
4.  Language production may be adequate but often lacks the characteristics of natural speech. 
5.  There are persistent, fossilized language errors.

See the Richards pamphlet linked to below for further information and suggestions of how to help learners overcome the plateau stage.

References and Further Reading

Yi, F. (2007) Yi, F. Plateau of EFL Learning: A Psycholinguistic and Pedagogical Study.
Richards, J.C, (2008)  Moving Beyond the Plateau, CUP