Mode
A mode is a particular modification of substance — every individual thing, idea, or event is a mode.
In depth
If substance is the ocean, modes are the waves. Every finite thing — you, this screen, a feeling of joy, a mathematical proof — is a mode of God/Nature. Modes do not exist independently; they exist "in" substance the way a wave exists in water. Some modes are infinite (like the laws of motion), but most are finite and temporary.
Why it matters
Understanding that you are a mode, not a self-standing entity, reframes identity. You are a temporary pattern in an infinite whole. This is not nihilism — Spinoza thinks understanding this leads to freedom and joy.