The investor or VC firm that organizes a specific round of funding, negotiates the valuation and terms, and usually writes the largest check.
Why it matters: Securing a lead investor is the hardest part of fundraising. Once a lead sets the terms, other 'follow-on' investors quickly fill the round.
In practice: Sequoia Capital issues a term sheet for $5M, acting as the lead, while smaller angels put in the remaining $1M.