70% is a community-term to describe compact keyboards in-between a 65% and tenkeyless in size, without a function-key row.
The layout is not precisely defined. It should have 5 rows of keys but could have 17 to 18 columns (in 1×1 key units).
It should have cursor keys and most or all keys from the nav cluster.
One or more Fn keys are often used to compensate for the lack of function keys.
70% keyboards typically have around 68-75 keys.