Negative Signal to Noise Ratio
Signal to Noise ratio (SNR) is a ratio of powers and hence it is always greater than or equal to zero, it cannot be negative.

On the other hand, very commonly SNR is expressed in decibel (dB) notation

$$ \text{SNR}_{\text{dB}} = 10 \log_{10}\left( \frac{\sigma_x^2}{\sigma_n^2 } \right) $$

where $\sigma_x^2$ and $\sigma_n^2$ are the signal and noise powers respectively. In which case, a $0$ dB SNR means that the signal power is equal to the noise power. And when the signal power is less than the noise power, then one gets a negative SNR in dB...