For Internet service providers, flat rate is access to the Internet all hour and days of the year (linear rate) and for all the customers of the telco operator (universal) at a fixed and cheap tariff.
A charge tariff is a class of linear rate, different from the flate rate, where the user is charged by the uploads and downloads (data transfers). The GPRS access to the Internet in some countries of Europe has no flat rates pricing, following the obsolete "metered mentality". Because of this, the users prefer using the fixed lines (with narrow or broadband access) to connect to the Internet.
A "wavy rate" is not a linear rate, because the Internet surfer pays the monthly fixed price to use the connection only during a certain range of hours of the day (i.e. only in the morning or, more typically, only at night).