The term 在野党 is straightforward, but its real-world meaning is entirely dependent on the political system in question. This is a critical distinction for any learner of Chinese.
In Democratic Systems (e.g., Taiwan, Japan, Western countries): In these contexts, 在野党 functions exactly like the English term “opposition party.” It is a legitimate and often powerful entity that holds the ruling party accountable, proposes alternative policies, and has a realistic chance of winning future elections to become the ruling party. News from Taiwan, for example, frequently discusses the actions of the 在野党 (e.g., the Kuomintang or KMT when the DPP is in power, and vice versa).
In Mainland China (PRC): This is where the concept diverges significantly. The People's Republic of China operates under a system led by the Communist Party of China (CPC). While other political parties exist (known as 民主党派, mínzhǔ dǎngpài, or “democratic parties”), they are not opposition parties. Their role is to participate in the political process under the leadership of the CPC. Therefore, calling them 在野党 is factually incorrect and politically sensitive. The correct term for them is 参政党 (cānzhèng dǎng), meaning “participating parties.” The term 在野党 is used in Mainland China's media, but almost exclusively when reporting on the politics of foreign countries.
Comparison to Western Culture: The Western ideal of a “loyal opposition” is a party that opposes the current government's policies but remains loyal to the state and its democratic framework. This aligns perfectly with the use of 在野党 in Taiwan. The critical difference is that this concept of a legitimate, power-seeking opposition does not apply to the internal political structure of Mainland China.