Flat yield curve
This means that the yield is the same for all maturities of funds.
For example the 1 year yield = 2 year yield = 3 year yield, and so on.
The relationships between the zero coupon yield, the forward yield, and the par yield depend on the basis on which the yields are quoted.
When all rates are quoted on an annual effective rate basis and the annual effective yield curve is flat, the zero coupon yield, the forward yield and the par yield are all the same, and all flat.