A System of Government by "Presbyters"
Presbyterianism is a method of governing a denomination. It's about structure, polity, and decision-making. It's very often the structure of denominations that trace their origins back to the Reformation in Geneva led by John Calvin and others, which inspired the Reformation in Scotland under figures such as John Knox.
Presbyterianism might be described as a system of government by "presbyters", and for us that means government by ministers, deacons, and elders gathered in the Kirk Session, the local Presbytery, and the annual General Assembly. You could have any number of courts in the pattern; nowadays we have three.
Presbyterian vs Reformed?
There is an overlap between Churches that call themselves "Presbyterian" and those that call themselves "Reformed" (with a capital "R"). The Reformed tradition is more of a theological brand, so it includes denominations whose governance structures are a bit different. However, in many places, including our Church, the two words are used virtually interchangeably.