
der Lacher
gag / laugh
A remark, action, or situation that makes other people laugh.
Sein Versprecher am Ende der Rede war der größte Lacher des Abends.
His slip of the tongue at the end of the speech was the biggest laugh of the evening.
laugh / gag
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gag / laugh
A remark, action, or situation that makes other people laugh.
Sein Versprecher am Ende der Rede war der größte Lacher des Abends.
His slip of the tongue at the end of the speech was the biggest laugh of the evening.

laugh
The sound made when laughing, or the act of laughing itself.
Ein lauter Lacher ertönte aus dem Nebenzimmer.
A loud laugh came from the next room.

to have the laughs on one's side
To win the audience's sympathy through wit or charm.
Mit seiner selbstironischen Bemerkung hatte der Redner sofort die Lacher auf seiner Seite.
With his self-deprecating remark, the speaker immediately had the laughs on his side.

laugher
A person who laughs or is known for laughing often. This meaning is rare.
Er ist ein herzlicher Lacher, seine gute Laune ist ansteckend.
He is a warm-hearted laugher; his good mood is contagious.
'Der Lacher' is derived directly from the verb 'lachen' (to laugh). The ending '-er' turns the action into a noun that describes either the result (laughter) or the cause of the laughter.
Der Witz war gut, aber der eigentliche Lacher kam erst danach.
The joke was good, but the real laugh came only afterward.
You distinguish the meanings through context: when it is about the cause, it is often 'ein Lacher' (a laugh/gag). When it is about the sound, it is often described with adjectives like 'laut' (loud) or 'herzlich' (hearty).
Sein Kommentar sollte ein Witz sein, war aber kein großer Lacher.
His comment was meant to be a joke, but it wasn't much of a laugh.
In the meaning of 'something that makes people laugh', the word is often used in fixed combinations, such as 'für einen Lacher sorgen' (to get a laugh) or 'ein Riesenlacher sein' (to be a huge laugh).
Der Komiker sorgte mit seiner Geschichte für viele Lacher im Publikum.
The comedian got many laughs from the audience with his story.
The expression 'die Lacher auf seiner Seite haben' (to have the laughs on one's side) is fixed. Only the possessive pronoun ('seiner', 'ihrer', 'meiner') changes depending on the person.
Nach dem ersten Witz hatte die Rednerin die Lacher auf ihrer Seite.
After the first joke, the speaker had the laughs on her side.
to laugh
Lachen generally refers to the activity or the sounds produced while laughing. Der Lacher, by contrast, usually refers more to a single audible utterance. When you are describing the process as a whole, lachen is appropriate; when you want to emphasize a single sound, ein Lacher is more precise.

Nach dem Witz kam aus der letzten Reihe nur ein kurzer Lacher.
After the joke, only a short laugh came from the back row.
Die Kinder lachen laut über den Witz.
The children are laughing loudly at the joke.