ETA Reduced Lambda

This is often abbreviated to "Eta Lambda".
This refers to a simplified syntax for entering simply types of Lambda formula.
Here is an example using the BYROW function and the SUM function.

=BYROW(array,LAMBDA(x,SUM(x))) 

'can now be abbreviated to
=BYROW(array,SUM) 

Here is an example using the BYROW function and the AVERAGE function.

=BYROW(array,LAMBDA(x,AVERAGE(x))) 

'can now be abbreviated to
=BYROW(array,AVERAGE) 

Example

The table shows 4 cities along the top and 6 months down the side.
If we want to average the four columns we could enter the AVERAGE function into cell "H3" and drag it down.

This works well although it requires 2 steps and has added 6 functions to the worksheet.

Using the BYROW function this can be achieved in one step.
In cell "H3" add the "BYROW" function instead and use this formula.
This formula is using the "eta reduced lamdba" syntax.

=BYROW(C4:F9,AVERAGE) 

Here is the same formula but without the "eta reduced" syntax.

=BYROW(C4:F9,LAMBDA(x,AVERAGE(x))) 

More Information

link - techcommunity.microsoft.com/blog/excelblog/new-aggregation-functions-groupby-and-pivotby/3965765 
link - spreadsheetweb.com/exploring-eta-lambda-in-excel-a-comprehensive-and-detailed-guide-with-examples/