Vocabulary is having knowledge of the meaning of words or phrases.

Having adequate vocabulary knowledge is necessary for comprehension.  Students with limited vocabulary will experience difficulty understanding and engaging with material.  Individuals entering school with limited vocabulary knowledge grow much more discrepant over time from peers with rich vocabulary knowledge.   Therefore, addressing vocabulary deficiencies is imperative for student success.

• Between grades 1 and 3, it is estimated that economically disadvantaged students' vocabularies increase by about 3,000 words per year and middle-class students' vocabularies increase by about 5,000 words per year.

• Children's vocabulary size approximately doubles between grades 3 and 7.

• An average student in grades 3 through 12 is likely to learn approximately 3,000 new vocabulary words each year, assuming he or she reads between 500,000 and a million running words of text a school year (Nagy & Anderson, 1984).

http://reading.uoregon.edu/big_ideas/voc

Vocabulary can be informally or formally assessed.