It means called, named, known as. The reason you couldn't find it is because it's the past participle of the verb to 'term'. The term (noun - meaning name/expression) is 'High Frequency Word'. If people use this term to refer to certain words, then these words are 'termed' 'High Frequency Words'.

"Termed" is the past participle of the verb "term." (See "term" in your dictionary.) The complete sentence is "the vocabulary words THAT ARE termed High Frequency Words." "Words that are termed"= words that are called or identified as High Frequency Words.

It means "given the term (name) of..."