Matrix rows notation I'm working with a set of $M$ vectors $ \{\mathbf{w}_i \in \mathbb{R}^N, \, i = 1, \ldots, M \}$. Since single vectors are usually considered as column vectors, I'm defining a matrix 

$$
\mathbf{W} = [\mathbf{w}_1, \ldots, \mathbf{w}_M] \in \mathbb{R}^{N \times M}
$$

by placing the vectors as matrix columns.

However, for some descriptions, I need to refer to the matrix rows.

Is there an elegant notation to refer to this matrix rows (preferably with less notation overhead)?
You can represent the rows of $\mathbf W$ by the  $M$-(column-)vectors $\mathbf w'_i :=\mathrm{col}_i\mathbf W^\mathsf T\;(i=1,\dots,N).$ There is no standard notation for this; I chose the prime notation for convenience. You would also need to state your chosen notation explicitly.
If you write
$$\mathbf{W} = [\mathbf{w}_1, \ldots, \mathbf{w}_M]^t \in \mathbb{R}^{M \times N}$$
then the rows of $\mathbf W$ are $\mathbf{w}_1^t, \ldots, \mathbf{w}_M^t$.