Beck et al. (2002) suggest that for instructional purposes, teachers should concentrate on Tier Two words. Most students come to school knowing Tier One words and Tier Three words should be taught as they occur in reading. Tier Two words are the words that can add most to students’ language knowledge.
Tier Two words include: (1) words that are characteristic of mature language users and appear frequently across a variety of contexts; (2) words that lend themselves to instruction and that can be worked with in a variety of ways so that students can build in-depth knowledge of them and their connections to other words and concepts; and (3) words that provide precision and specificity in describing a concept for which the students already have a general understanding (Beck et al., 2002).
(For more information see http://www.mikemcmahon.info/vocab.htm)