Abeam is a term used in aviation to describe the positioning of an aircraft directly across from or over a NAVAID (navigational aid) or any other type of fix. A fix can be an airport, tower, waypoint, or any notable naturally or manmade geographical object.
Most commonly “Abeam” will be used to describe a pilots position directly across from or over a NAVAID. A NAVAID can be a VOR (very high frequency omni-directional range beacon), NDB (non-directional beacon), or GPS waypoint. In order to be abeam a NAVAID you have to have instrument confirmation that you next to it. In a VOR this is commonly shown by the shaking of the CDI needle. The CDI needle is the need in an aircraft VOR instrument that shows the direction to, from, left, or right of the NAVAID.
With a GPS waypoint being abeam a fix would be show by an indication on the GPS screen and with an NDB it would be shown on the ADF (automatic-direction finder) instrument with an arrow point to the side of an airplane or completely reversing in direction. Abeam a fix can be noted either visually or by instruments. So depending on the type of aircraft and equipment available in that aircraft whether abeam a fix or not will be found different ways.
A pilot can be asked to perform special instructions or report their position to traffic control (ATC) when abeam a fix in cases where the controller is providing positive separation to aircraft or when performing special approaches. When a pilot is holding over a fix or NAVAID then being abeam a point plays an important role. In order to fly a correct holing pattern the pilot must be able to track a course to and from the NAVAID or fix correctly. They must make their turns to perform the holding patterns at the right times and being abeam the NAVAID/fix determines those turns.
Also when a pilot is traveling using the help of an airway they might be required to change courses to follow a different NAVAID. This course change is usually denoted by being abeam the NAVAID.