Short answer. A partner's interest is his share of the profits and surplus of the partnership — his financial stake in what the business earns and in whatever is left after debts are paid. Article 1812 defines it in exactly those terms. It is a property right the partner owns, distinct from the partnership's own assets.
What the law says
A partner's interest in the partnership is his share of the profits and surplus.
Civil Code, Article 1812 — A Partner's Interest. Read the full provision →
Profits and surplus — the two components
Article 1812 puts it in one line: A partner's interest in the partnership is his share of the profits and surplus. Those are two distinct things. Profits are the gains the business produces as it operates — the share of the earnings the partner is entitled to receive, usually while the firm is a going concern. Surplus is what is left of the partnership's assets after everything it owes has been paid — the creditors settled and the partners' contributions accounted for — when the partnership is wound up. So a partner's interest looks in two directions: to the income of the business now, and to the residue that remains for the partners once the business is closed out.
What the interest is not
It is important to see what this interest does not include. A partner's interest is not ownership of any particular asset of the partnership. Even though the partners together own the firm's property, no single partner can point to the delivery truck, the inventory or the office and call it his; those belong to the partnership and are held for its purposes. A partner's right in specific partnership property is a different kind of right, tied to partnership business, and it cannot be dealt with as freely as personal property. The interest under Article 1812 is the abstract, money value — the share of profits and surplus — not a slice of the physical assets themselves.
It can be transferred — but a transfer makes no new partner
Because the interest is essentially a property right to money, a partner may generally assign it or use it as security, and a personal creditor may reach it to satisfy the partner's own debt. But a crucial limit follows from the nature of a partnership, which rests on the mutual trust of the partners. A person who buys or is assigned a partner's interest does not thereby become a partner. The assignee is entitled to receive the profits and surplus that would have gone to the assigning partner, but gains no right to take part in management, to inspect the books, or to interfere in how the business is run. Partnership is a relationship you are admitted into, not one you can buy your way into.
Why the definition matters in practice
This narrow definition decides real disputes. When a partner dies, retires or is bought out, what is valued and paid is this interest — the share of profits and surplus — not a claim to specific company property. When a partner's personal creditor comes calling, it is this interest, not the firm's assets directly, that the creditor can pursue. And when partners quarrel over money, the line between the firm's property and each partner's interest often settles who is entitled to what. Because valuing a share of profits and surplus can be contentious, especially at dissolution or on a partner's death, the partnership's records and its agreement should be reviewed carefully, ideally with counsel, before any payout is agreed.