If I am understanding you:
x : {{__} ..}
See Repeated for more information and additional options. Also see RepeatedNull while you're there.
Make sure you understand BlankSequence and Pattern as well.
Here is a breakdown of the expression above. First let us view the FullForm which is as close to the way Mathematica sees it as possible:
FullForm[ x:{{__}..} ]
Pattern[x,
List[
Repeated[
List[
BlankSequence[]
]
]
]
]
This expanded form is useful to remove any ambiguity in Mathematica's parsing.
Therefore from the inside out we have (short form : long form : description):
__ : BlankSequence[] : one or more arguments with any head
{ } : List[ ] : inside the head List
.. : Repeated[ ] : one or more arguments matching the given pattern
{ } : List[ ] : inside the head List
x: : Pattern[x, ] : a unique expression that matches the given pattern, named x
Pay attention to this last point: naming the pattern changes the way it behaves, such that it represents a unique expression. Consider this superficially similar pattern:
x : {{a__} ..}
This will only match e.g. {{1, 2}, {1, 2}, {1, 2}} but not {{1, 2}, {3}, {4, 5, 6}} because by naming the first sequence 1, 2 all other sequences must be identical. Simply matching the pattern a__ independently is not enough.