the anti particle of the particle Electron

A Positron is the anti particle of the particle Electron.

which is an electron with positive electric charge

For example, the particle conjugated to the electron with the electron's negative electric charge is positron, which is an electron with positive electric charge.

an elementary particle of antimatter

A positron is an elementary particle of antimatter.

An electron travelling backwards in time

An electron travelling backwards in time is what we call a positron.

an antiparticle with a positive charge

The stable leptons are the electron (with a negative charge) and the positron (an antiparticle with a positive charge).

a positively charged particle

Well, a positron's a positively charged particle, but a positron isn't a proton.

the antimatter counterpart of the electron

Scientists know that every matter particle has an antimatter partner particle with opposite charge; for instance, the antimatter counterpart of an electron is a positron.

an antimatter counterpart called the positron

The electron has an antimatter counterpart called the positron.

the antiparticle of the electron – a particle with the same mass but opposite charge

The positron is the antiparticle of the electron – a particle with the same mass but opposite charge.