Freebase(0.00 / 0 votes)Rate this definition: In structural geology, a syncline is a fold with younger layers closer to the center of the structure. A synclinorium is a large syncline with superimposed smaller folds. Synclines are typically a downward fold, termed a synformal syncline; but synclines that point upwards, or perched, can be found when strata have been overturned and folded.