A waveform is the shape of a cycle in a tone.
In electronic music, a common term is waveform. The waveform describes either the shape of a single cycle or a full sequence of cycles, also called sample.
The shape of a waveform defines the sound of a tone.
The four waveforms above are very common for building synthesisers, but usually waveforms for more natural sounds are way more complex. They are in fact typically that complex you cannot visually spot them. The following image is showing a short section of the waveform of a natural string sound.
We talk about the perceived pitch of complex waveforms like this. The perceived pitch is often, but not always the lowest frequency in a sound, also called fundamental frequency. Two different waveforms played at the same frequency can be perceived as two different pitches.