Numbers of the form $\sum_{i=1}^ni=1+2+3+4+...+n$.
Triangular numbers are named as such as they represent the number of dots in an equilateral triangle with $n$ dots a side. The sequence (OEIS sequence A000217) has the closed-form $$T_n=\sum\limits_{i=1}^ni=1+2+...+n=\frac{n\cdot(n+1)}2={n+1\choose2}$$
Use this tag solely if your question is about a property concerning this sequence.
