General term for a jar, often a glass or plastic container with a lid. Less common than sense 1 in everyday Japanese; 瓶 (びん) is more typical for small jars.
I store cookies in a jar.
I put the cooked rice into the warmer to keep it warm.
I put my lunch in a thermal container and take it with me.
I keep the rice warm in an electric rice warmer so it doesn't get cold.
I kept the soup warm in the thermos.
Use 台 when counting a ジャー as an appliance, e.g. 炊飯ジャー.
Borrowed from English 'jar', with a specialized meaning for thermal containers in Japanese.