Thermos is a trademarked name for a make of vacuum flask. This is rather like calling a vacuum cleaner a "Hoover" (a common word for it in the UK, which has also become a verb for the act of cleaning with a vacuum cleaner). They are both trademarks and should be capitalized.
So a Thermos is vacuum flask in the form of a commercial product.
"Particular" here just means an individual example rather than one with features that sets it aside from the norm. A Thermos is a vacuum flask in the form of a commercial product.
The big OED gives this definition:
"A registered trade term noting a flask, bottle, or the like capable of being kept hot or cold by the device (invented by Sir James Dewar) of surrounding the interior vessel with a vacuum jacket to prevent the conduction of heat. Hence (freq. with small initial) applied loosely to any vacuum flask."