"Lane Splitting" refers to the act of riding a motorcycle or similar vehicle between lanes in the same direction traffic. It also refers to the act of overtaking or passing other vehicles, without regard for lane boundaries.
In Nevada, it's illegal for a motorcycle or moped to move between moving or stationary vehicles occupying adjacent traffic lanes. NRS 486.351(1). Similarly, cyclists must respect the right of other vehicles to occupy full lanes of travel, and thus avoid passing or overtaking vehicles within the same travel lane. NRS
With regard to driving "abreast", NRS 486.351(3) permits cyclists to operate side by side within a single lane, provided each driver has consented to the activities. Similar to Nevada's Mandatory Helmet Law, this activity is treated differently from state to state.