Eviction is the legal method of removing a tenant. A tenant is someone who is paying rent and/or who has a lease. Ejectment is the legal method of removing someone who is not a tenant, in other words, usually someone who is staying without a lease and not paying rent to the landlord. I would assume that you are the legal tenant on the property and that you have invited your girlfriend to stay with you without your landlord's permission or against your landlord's permission. Your girlfriend can't be evicted because she's not on the lease, therefore she must be ejected. Eviction and ejectment are both ways of kicking someone out, they just have a subtle legal difference in that eviction involves tenants and ejectment involves non-tenants.
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I write only to point out that something would seem to have precipitated and predated the letter of ejectment. I would attempt to get to the root of that issue before the lawsuit for ejectment is filed. Seek legal help.
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