labour US, labor ['le?b?]
nBy collins dictionary
1. (Business / Industrial Relations & HR Terms) productive work, esp physical toil done for wages
Because while the word has meaning (1) in general, there is a subset of (1) for which it is used especially often. "Labor" can mean any sort of productive work, but it's often used to mean specifically physical toil done for wages.
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soheil1What is in esp-part, which is not in the previous part?Nothing. That is, the "esp...." portion will always be simply a narrower definition, not a different meaning. "Esp." just tells you about how the word is most frequently used.