A spotter of something such as trains or aeroplanes is someone whose hobby is watching and finding out about them.
[British]
I was a devoted train spotter.
spotter in British English
(ˈspɒtə)
noun
1.
a.
a person or thing that watches or observes
b.
(as modifier)
a spotter plane
2.
a person who makes a hobby of watching for and noting numbers or types of trains, buses, etc
a train spotter
3. military
a person who orders or advises adjustment of fire on a target by observations
4.
a person, esp one engaged in civil defence, who watches for enemy aircraft
5. US informal
an employee assigned to spy on his or her colleagues in order to check on their honesty
6. cinema
a.
a person who checks against irregularities and inconsistencies
b.
a person who searches for new material, performers, etc
spotter in American English
(ˈspɑtər)
noun
a person or thing that spots
; specif.,
a.
a person whose work is removing spots, etc. in dry cleaning
b. US
a person hired to watch for dishonesty, etc. among employees, as in a store
c.
a person who watches for, and reports, enemy aircraft
d.
a person who determines, as for a gun crew, the position of a target and the closeness to it of the projectiles fired
e. US
a football coach's assistant in the stands who reports on the plays to the bench by phone; also, an assistant to a sports announcer who helps identify the players for the announcer
Examples of 'spotter' in a sentence
spotter
Some fleets use spotter aircraft despite their being banned.
Times, Sunday Times (2012)
Finally, a rescue team in a spotter plane found the crash site.
The Sun (2006)
Four Italian coastguard vessels and a spotter plane were joined by at least nine passing merchant ships helping with the rescue operations yesterday.
Times, Sunday Times (2015)
The fleet relied on spotter planes for weather forecasts in the surrounding seas, and these aircraft reported no signs of any bad weather.
Times, Sunday Times (2012)
Since the introduction of sonar, radar and spotter planes, commercial fishing has caught bluefin tuna faster than nature can replace them.
Times, Sunday Times (2010)
The UFO spotter snapped a similar, lone spaceship days before.
The Sun (2008)
All related terms of 'spotter'
bird-spotter
a bird-watcher
plane spotter
a person who observes , photographs, and catalogues aircraft as a hobby
spotter plane
an aircraft used in observing enemy movements or the accuracy of fire on a target
train spotter
a person who collects the numbers of railway locomotives
talent spotter
a person whose occupation is the search for talented artists , sportspeople, performers, etc, for engagements as professionals
trainspotter
A trainspotter is someone who is very interested in trains and spends time going to stations and recording the numbers of the trains that they see.