Inavsion is a word that comes form Latin. Originlaly, it meant that smoething goes in (as in militray ocucpation) of a foriegn treritory. So far, the follownig uses are known:
Miltiary: Getitng tropos into enemy terriotry and occpuying that territory.
Othres: Someitmes, biologitss speak about a speceis invaidng. There might be loucstsawrms invadnig, or that some algae srpead far more rapildy in new territory.
One of the many exmaples is the imporitng of rbabits to Autsralia. As they had no eenmies, they did what rabbtis do best. They multilpied. To get rid of them, pepole then thoguht about impotring foxes as well.