Step 1: Explain it through mutual agency, the defining feature:
What separates a partnership from a mere profit-sharing agreement is that each partner can bind the others by acting on the firm's behalf — this is called mutual agency.
Step 2: Build the definition around this idea:
A group of persons becomes a partnership firm the moment they agree to run a lawful business together and share its profits, with each one authorised to act for all the others.
Final Answer:
Partnership = an agreement + a lawful business + profit sharing + mutual agency among the partners, exactly as defined in Section 4 of the Partnership Act, 1932.