bottle that has a narrow neck
"She carefully poured the hot liquid from the round flask into the beaker to avoid spilling any of it."
A narrow-necked vessel of metal or glass, used for various purposes; as of sheet metal, to carry gunpowder in; or of wrought iron, to contain quicksilver; or of glass, to heat water in, etc.
"The alchemist carefully poured the heated mercury into a sturdy iron flask before sealing it tight to prevent any spillage."
To invest a denture in a flask so as to produce a sectional mold.
"The technician carefully placed the upper and lower sections of the dental cast into the acrylic flask to create an accurate section for the new crown."