A Tk (or rather Ttk) widget that stacks a bunch of widgets on top of each other in a window, with some tabs at the top to allow users to select between the widgets. A common idiom in Windows's Properties dialogs.
A notebook may be used as a pages manager[L1 ] by using a style to turn off the tabs, like this (by Joe English):
style theme settings default {
style layout Plain.TNotebook.Tab null
}
ttk::notebook $nb -style Plain.TNotebook