An aketon or gambeson is a type of padded garment worn over or over armour or as a stand alone armour in its own right. There is some disagreement of the use of the term modernly but in period both seem at times interchangeable. In the 15th jack and arming jacket becomes used in England but it seems it is just more specific terms for similar items, with changes of time and fashion added in.
Often this was the bare requirement of armour for footmen. Used with plate/mail it adds padding to dissipate impact and avoid chafing and bruising from the armours movement.
Here is an example under armour- it is sews with diagonal channels though it is visible between the tassets of the armour.
http://www.southtower.on.ca/armour/imag ... Armour.JPG
Without plate/mail armour.
http://www.medievaltymes.com/courtyard/ ... etail7.gif