What is a microsome?
The first quote is correct. 'Microsome' is more of a lab term. This is because, as said they are found (re-formed) after centrifugation and as such aren't seen in an intact cell. Differential centrifugation is a technique used to separate cellular constituents into fractions. To quote,


  The microsomal fraction is the pellet produced when the postmitochondrial supernatant is centrifuged at 50,000g for 60 minutes. Microsomes are small vesicles enclosed by a biological membrane. During centrifugation, the endoplasmic reticulum is fragmented into microsomes 


The microsomal fraction may also contain pieces from the plasma membranes apart from those from the ER.