In an ideal transformer, all of the flux generated in the primary winding will be linked to the secondary winding.
In such a transformer, the efficiency will be maximum. Modern Transformer cores use high silicon steel with high permeability and winding which are placed close to each other to achieve a high coefficient of coupling.
However, in actual transformers, some of the flux built in the primary winding is not linked to the secondary. This flux which is not linked to the secondary is known as the leakage flux. The leakage flux causes a drop in secondary voltage.
For the purpose of calculation, the leakage flux is assumed to be an inductance connected in series with the primary winding which causes a drop in the applied voltage in the primary. This is known as the leakage inductance.