Copyright for a written work expires some years after the author's death; the length of time under copyright law varies from country to country. Wikipedia and other sites use a variety of licencing models which allow the material to be copied and the conditions that apply to the use of the material.
But regardless of whether something is still copyrighted or not, open source or not, freely available or not, if it is quoted, cited, copied or otherwise repeated in a text, if the source is not cited then yes, it is plagiarism.
When something passes into "common knowledge" then it can be written without having to find a quotable source. But there are also specific situations where something that might normally be considered "common knowledge" needs to be referenced to a source, such as student work where values, definitions, etc., are expected to be researched for accuracy and should also therefore include a reference.
There may also be the case where the value/definition is used in a way that is outside the norm, e.g. a definition that is unusual in the mainstream but is used in a specific way for a specific context, or a value that is based on some work that extends the precision beyond what is considered to be the norm.
The criteria remains, that if someone else wrote it and you copy it then not crediting the original author/source is plagiarism - academic theft.