Define trade union and explain the registration machinery and its legal effects.

The trade union = the workers' continuous association — the members' collective for the advancement of the terms-and-conditions of the employment (the strength-through-unity instrument). The concept's legal faces: the union as the bargaining vehicle (the collective's representation — Unit 5's agent); the union as the legal person (the registration's certificate — the personality's birth: the sue-and-be-sued capacity, the property holding, the agreement's party); the union as the political-and-social voice (the labour's representation — the policy's advocacy). The association's right (the union's constitutional-and-international anchors): the freedom of association (the Constitution's guarantee); the ILO's core pair (C87-C98 — the C98 ratified; the C87's Nepali gap the standing debate); the anti-union-discrimination protections (the dismissal's bar for the union activity; the office bearers' security faces). The movement's history (the context's paragraph): the international line — the combinations' criminalisation (the 18th-19th centuries' conspiracy doctrine) → the legality's struggle → the consolidation (the national centres; the internationals) → the ILO's institutionalisation → the modern decline-and-renewal (the density debates; the gig-and-informal organising). The Nepali line: the pre-1951 suppression; the 1951 opening (the unions' legal birth); the Panchayat era's controls (the government-sponsored structures); the 1990 revolution (the independent unions' explosion — the federations' multiplicity and politicisation); the modern face (the party-aligned federations; the informal-and-migrant organising's challenge).