Sentence:  Before discussing ways to counter <WSD>zero-day</WSD> attacks, let's begin with the definition of what a <WSD>zero-day</WSD> attack is. A <WSD>zero-day</WSD> attack is one that the vendor does not yet know about or hasn't been fixed. 
Meaning of the word marked with <WSD> tags:  (of exploit, its threat, or attack) benefiting from newly found and yet unpatched or unmitigated flaw in software or hardware; using zero-day vulnerability