One decilitre is exactly 100 millilitres. The unit is close to unknown in English-speaking kitchens and completely standard in Nordic ones, where it is the primary measure for both liquid and dry ingredients.
About the units
The decilitre is a tenth of a litre, from the Latin decimus. It is one of the metric prefixes SI keeps but discourages, and like the centimetre and the hectogram it survived because it fits a real human scale that the preferred steps of a thousand miss. A decilitre is roughly a small glass, and Nordic measuring jugs are graduated in it from 1 dl upwards. Swedish, Norwegian and Danish recipes specify flour, sugar, rice, milk and stock in deciliters as a matter of course, which is unusual — most metric countries measure dry ingredients by weight and reserve volume for liquids.
The exact factor
The factor is exactly 100, a two-place shift. 2.5 dl is 250 ml; 400 ml is 4 dl. The family: 10 dl to the litre, 10 cl to the decilitre, 100 ml to the decilitre. For anyone converting Nordic recipes, the two numbers to hold are that 1 dl is 100 ml and that a US cup is 2.37 dl — the second being the one that makes recipe conversion awkward, since no round number of deciliters corresponds to a whole number of cups.
Where you meet this conversion
Nordic cooking and baking above all. A Swedish bread recipe calling for 12 dl of flour is asking for 1.2 litres by volume, which is around 720 g depending on how the flour is packed — and that variability is exactly why weighing has been gaining ground even in Sweden. Outside cooking the decilitre appears in Nordic school mathematics and in some medical contexts, where blood values are reported per decilitre in some countries and per litre in others.