der Meter
meter
The base unit for measuring length in the metric system, equal to 100 centimeters.
Der Tisch ist zwei Meter lang.
The table is two meters long.

meter
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meter
The base unit for measuring length in the metric system, equal to 100 centimeters.
Der Tisch ist zwei Meter lang.
The table is two meters long.

When 'Meter' is used as a unit of measurement with a number greater than one, it remains unchanged as 'Meter' in the nominative, accusative, and genitive. The form 'Metern' is used only in the dative after a preposition.
Der Schrank ist zwei Meter hoch.
The wardrobe is two meters high.
In everyday life, especially on construction sites or when doing DIY, 'der Meter' is often used as a colloquial term for a measuring tool such as a folding ruler or a tape measure.
Kannst du mir bitte mal den Meter reichen? Ich muss das Brett abmessen.
Could you please pass me the measuring tool? I need to measure the board.
'Meter' is the basis for units of area and volume. For area, you use 'der Quadratmeter' (square meter, m²), and for volume, 'der Kubikmeter' (cubic meter, m³).
Unsere Wohnung hat 80 Quadratmeter.
Our apartment has 80 square meters.
You can easily distinguish the two meanings: if a number comes before it, the unit of measurement is meant. If an article such as 'der' or 'den' comes before it and it is about an action, the measuring tool is meant.
Die Brücke ist 500 Meter lang.
The bridge is 500 meters long.
kilometer
'der Meter' and 'der Kilometer' are both units for measuring length. The difference is the size: one kilometer is 1,000 meters. 'Meter' is used more for shorter lengths, for example with a room, a table, or a person's height. 'Kilometer' is used more for larger distances, for example between cities or on a trip. Learners may confuse the words because both measure length. The important point is: for small or medium lengths, you usually say 'Meter'; for long distances, you usually say 'Kilometer'.

Der Flur ist nur drei Meter lang.
The hallway is only three meters long.
Bis zum nächsten Bahnhof sind es zwei Kilometer.
It is two kilometers to the next train station.