Accuracy
The share of predictions a model gets right, usually written as a percentage. It's the simplest way to summarize how well a classifier is doing.
Think of It Like This
It's a report-card percentage — 90% accuracy means 9 out of 10 calls were correct.
Accuracy is just correct predictions divided by total predictions. It's a great headline number when your classes are balanced — but it can badly mislead when they aren't. If 99% of emails are legitimate, a model that flags nothing as spam still scores 99% accuracy while catching zero spam, which is why teams pair accuracy with metrics like precision and recall.