A simple, empirical signature of a first order phase transition in atomic nuclei is presented, the ratio of the energy of the 6 + level of the ground state band to the energy of the first excited 0 + state. This ratio provides an effective order parameter which is not only easy to measure, but also distinguishes between first and second order phase transitions and takes on a special value in the critical region. Data in the Nd-Dy region show these characteristics. In addition, a repeating degeneracy between alternate yrast states and successive excited 0 + states is found to correspond closely to the line of a first order phase transition in the framework of the Interacting Boson Approximation (IBA) model in the large N limit, pointing to a possible underlying symmetry in the critical region.The study of structural evolution in atomic nuclei has witnessed significant developments in recent years. One of the most important has been the discovery of empirical evidence [1,2] for quantum phase transitions (QPT) in the equilibrium shape as a function of nucleon number. This has led to the proposal [3,4] and empirical verification [5,6] of a new class of models, called critical point symmetries (CPS). These, in turn have spurred an abundance of experimental searches [7,8,9] for nuclei satisfying the predictions of these CPS as well as investigations into the presence of quasidynamical and partial dynamical symmetries at the critical point [10,11].In contrast to the usual thermodynamic phase transitions [12], QPT occur at zero temperature as a parameter in the Hamiltonian is varied [13]. They are attracting much attention in a variety of physical systems, including Josephson-junction arrays and quantum Hall-effect systems [14]. The transition from BardeenCooper-Schrieffer (BCS) pairing correlations [15] to Bose-Einstein condensation (BEC) as a result of increasing the strength of the pairing interaction in ultracold alkali atoms has been recently seen experimentally [16,17,18]. In nonrigid polyatomic molecules, a transition from rigidly-linear to rigidly-bent shapes has also been studied [19]. A relationship between QPT and quantum entanglement has also attracted much attention [20,21]. Central to this active field is the search for simple empirical signatures of QPT and the identification of their order. The properties of QPT in atomic nuclei are therefore of quite broad interest, especially since nuclei are finite-body systems in which the number of constituents can be varied experimentally and theoretically.It is the purpose of this Letter to identify a new, easyto-measure, observable in one such system, atomic nuclei, that acts as an order parameter for QPT in nuclei and which can distinguish first and second order phase transitions. Further, we will show that a particular value of this observable identifies the entire critical region, even for structures not satisfying a specific CPS. Finally we use this value to point to the possible existence of a heretofore undefined new symmetry at the critical point.In the co...