1. keywords
the words (or n word sequences) which are significantly more frequent in a specialised corpus than in a "reference corpus"
generally, the reference corpus will be a large mixed corpus (like the BNC)

2. "keyness"
the probability that the frequency found in the specialised corpus is due to chance eg
100 occurrences in specialised corpus of 100,000 words
1000 occurrences in reference corpus of 100,000,000 words
occurrence is 100 times more frequent in specialised corpus
p< less than 1% probability
typically calculated using either
chi squared
log likelihood

3. stop lists
lists of very common words (typically "function words" - articles, prepositions, conjunctions, pronouns, auxiliary verbs, negation elements ...)
excluded from keyword lists

4. POS tagging Treetagger XML version another DETERMINER word NOUN
<w pos="det">another</w> <w pos="noun>word</w>