Here's one way to start...

dataset = Dataset[{{195, 2.3}, {198, 6.4}, {204, -3.1}}];
datasetWithHeader = dataset[All, Association @@ Thread[{"x", "y"} -> #] &]

Then you could continue:

datasetWithHeader[All, <|"x" -> Quantity[#x, "Centimeters"], "y" -> Quantity[#y, "Volts"]|> &]

Very nice

Thanks very much.

Regarding this instruction you wrote:

dataset[All,Key["c"]/*<|"ctotal"->Total,"clength"->Length|>] 

Can you show me how to write it in a "FullForm" notatation, With explicit RightComposition? I am not so familiar with this infix shortcut and don't fully understand which expression is the argument of which function.

Another question: You showed how to apply a function (Total and Length) on the values in each row.

Suppose I want to add 3 to each value, or multiply each value by 2, in column a, how to do it?

Another way, if you start with raw numbers:

list = {{195, 2.3}, {198, 6.4}, {204, -3.1}}

Note that both Thread[{"x", "y"} -> {195, 2.3}] and Thread[{"x", "y"} -> #] &[{195, 2.3}] return

{"x" -> 195, "y" -> 2.3}

Map the pure function Thread[{"x", "y"}->#]& on every sub-list:

Map[Thread[{"x", "y"} -> #] &, list]
(*{{"x" -> 195, "y" -> 2.3}, {"x" -> 198, "y" -> 6.4}, {"x" -> 204, "y" -> -3.1}}*)

and replace the head of each sub-list with that of Association:

Apply[Association, Map[Thread[{"x", "y"} -> #] &, list], {1}]
(*or*)
Association @@@ Map[Thread[{"x", "y"} -> #] &, list]
(* {<|"x" -> 195, "y" -> 2.3|>, <|"x" -> 198, "y" -> 6.4|>, <|"x" -> 204, "y" -> -3.1|>} *)

Only thing left is to make this a Dataset:

Dataset[%]

Written as a single instruction:

Association @@@ Map[Thread[{"x", "y"} -> #] &, list] // Dataset