Is copying standard definitions from e.g. ICD without quotes considered verbatim plagiarism In  textbooks published by my faculty staff members, Authors often include definitions from WHO or ICD using passive speech (... is defined as ...) 


Is this legal?
Is copying these standard definitions without quotation marks considered as
verbatim plagiarism ?
Definitions straddle a line between things that clearly require attribution and things that are "common knowledge" and do not require attribution.  For example, if I write on my own that:


  influenza is a viral disease of the respiratory system that causes fever, aches, and other symptoms


then I need not attribute any source, because this is common knowledge.  On the other hand, if I copy verbatim the Merriam-Webster definition:


  a common illness that is caused by a virus and that causes fever, weakness, severe aches and pains, and breathing problems


then even though it's almost identical, I need to attribute the source because they aren't my own words.  For such simple and widely accepted information, that attribution might not need a formal reference or even quotes, e.g., 


  The Merriam-Webster dictionary defines influenza as a common illness that is caused by a virus and that causes fever, weakness, severe aches and pains, and breathing problems.


Quotes would be better, but the attribution is still clear, so it's a minor problem at worst.  

Now, how does this apply to the situations that you have described?


First and foremost, if the definitions are being taken from standard references, the link to standard references is valuable for students to know, so from a pedagogical point of view they should be included even if it weren't strictly necessary.
If a large number of definitions are being presented specifically as the professor's own work, when they are not, then it is clearly a case of plagiarism (albeit a particularly banal and pointless one).
If, on the other hand, they are being presented more generally as "information about the subject", then it is more of a grey area.  They should be attributed, but informal material like course notes is often treated quite sloppily in practice, and it is likely to be perceived as a shortcoming to be corrected rather than a serious issue of plagiarism.