Malware
Malware (a portmanteau for malicious software) is any software intentionally designed to cause damage to a computer, server, client, or computer network. Malware does the damage after it is implanted or introduced in some way into a target's computer and can take the form of executable code, scripts, active content, and other software. The code is described as computer viruses, worms, Trojan horses, ransomware, spyware, adware, and scareware, among other terms. Malware has a malicious intent, acting against the interest of the computer user—and so does not include software that causes unintentional harm due to some deficiency, which is typically described as a software bug.
Fileless malware
This synonymy is however incorrect. Although the aforementioned behavioral execution environment is the same, in both cases i.e. both malware variants are executed in system memory, the crucial differentiation is the method of inception and prolongation. Most malware’s infection vector involves some writing to the hard disk, in order for it to be executed, whose origin could take the form of an infected file attachment, external media device e.g. USB, peripheral, mobile phone etc., browser drive-by, side-channel etc.