In this paper, we consider an inverse problem for a nonlinear wave equation with a damping term and a general nonlinear term. This problem arises in nonlinear acoustic imaging and has applications in medical imaging and other fields. The propagation of ultrasound waves can be modeled by a quasilinear wave equation with a damping term. We show the boundary measurements encoded in the Dirichlet-to-Neumann map (DN map) determine the damping term and the nonlinearity at the same time. In a more general setting, we consider a quasilinear wave equation with a one-form (a first-order term) and a general nonlinear term. We prove the one-form and the nonlinearity can be determined from the DN map, up to a gauge transformation, under some assumptions.This theorem shows the unique recovery of the one-form b(x) and the nonlinear coefficients β m+1 for m ≥ 1, from the knowledge of the DN map, up to a gauge transformation, under our assumptions. On the one hand, without knowing the potential h(x), one can recover b(x) up to an error term 2̺ −1 d̺, where ̺ ∈ C ∞ (M ) with ̺| ∂M = 1. On the other hand, with the assumptions on h(x), we can show this error term is given by some ̺ ∈ C ∞ (Ω) with ̺| ∂Ω = 1, which corresponds to a gauge transformation, for more details see Section 2. We point out it would be interesting to consider the recovery of the potential h(x) from the DN map but this is out of scope for this work.The inverse problems of recovering the metric and the nonlinear term for a semilinear wave equation were considered in [54], in a globally hyperbolic Lorentzian manifold without boundary. The main idea is to use the multi-fold linearization and the nonlinear interaction of waves. By choosing specially designed sources, one can expect to detect the new singularities produced by the interaction of distorted plane waves, from the measurements. The information about the metric and the nonlinearity is encoded in these new singularities. One can extract such information from the principal symbol of the new singularities, using the calculus of conormal distributions and paired Lagrangian distributions. Starting with [54,53], there are many works studying inverse problems for nonlinear hyperbolic equations, see